Estimated Read Time: 25 minutes | Target Exam: USMLE Step 1 & Step 2 CK | Difficulty: Mixed (Beginner to Advanced)
How to Use These USMLE Sample Questions
This is not a list of trivia questions. Every single question in this guide is written in authentic USMLE vignette format — the same structure the NBME uses on the real exam. Each question includes a clinical scenario, five answer choices, the correct answer, and a detailed explanation that teaches you how to think through the problem, not just what the answer is.
Before you begin, here is the most important rule for using sample questions effectively:
Read every explanation — especially for questions you answered correctly.
Getting a question right for the wrong reason is one of the most dangerous patterns in Step 1 preparation. You might guess correctly on a pharmacology question today and face the same concept from a different angle on exam day — and miss it. Deep explanation review transforms sample questions from a testing tool into a teaching engine.
Work through each section below as if you’re sitting for the actual exam. Cover the answer choices, read the vignette, commit to an answer — then check the explanation. This active engagement is what drives retention.
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What Makes These Questions Different from Generic Practice Tests
Most online USMLE sample question lists fall into one of two traps: they’re either too simple (basic recall questions that the NBME would never actually use), or they’re poorly explained (just “B is correct” without any reasoning).
These questions are built around the three-layer model the NBME uses for every question:
- Layer 1 — The clinical presentation: What the patient looks like, feels like, what their labs show
- Layer 2 — The underlying mechanism: The pathophysiology, biochemistry, or pharmacology driving the presentation
- Layer 3 — The clinical decision: What to do, what to expect, what went wrong
Mastering all three layers — not just recognizing a diagnosis — is what separates a 230 from a 260.
SECTION 1: Biochemistry & Genetics (Questions 1–8)
QUESTION 1
A 3-day-old male infant is found to have an elevated phenylalanine level on newborn screening. His parents are healthy and unrelated. He is started on a phenylalanine-restricted diet. At age 6 months, despite strict dietary compliance, he continues to have progressive neurological deterioration, including hypotonia, seizures, and developmental delay. His phenylalanine levels are only mildly elevated. What is the most likely underlying defect?
A) Phenylalanine hydroxylase deficiency
B) Tyrosine hydroxylase deficiency
C) Dihydropteridine reductase deficiency
D) Homogentisate oxidase deficiency
E) Fumarylacetoacetate hydrolase deficiency
✅ Correct Answer: C — Dihydropteridine reductase deficiency
Detailed Explanation:
This is a malignant hyperphenylalaninemia question — a variant of PKU that the NBME tests specifically because it behaves differently and requires a different treatment.
Classic PKU: Phenylalanine hydroxylase (PAH) deficiency → phenylalanine can’t convert to tyrosine → accumulation → intellectual disability, musty odor, fair skin/hair. Responds well to dietary phenylalanine restriction.
This patient’s clue: Diet is compliant, phenylalanine is only mildly elevated, but he has severe progressive neurological disease. This means the problem is NOT just phenylalanine accumulation — it’s a cofactor problem.
Phenylalanine hydroxylase requires tetrahydrobiopterin (BH4) as a cofactor. BH4 is also required for:
- Tyrosine hydroxylase (dopamine synthesis)
- Tryptophan hydroxylase (serotonin synthesis)
Dihydropteridine reductase (DHPR) normally regenerates BH4 from its oxidized form. Without it, BH4 is depleted despite adequate PAH enzyme → no dopamine, no serotonin → severe neurological devastation independent of phenylalanine levels.
Treatment: BH4 supplementation + L-DOPA + 5-hydroxytryptophan (precursors to dopamine and serotonin). Dietary restriction alone is insufficient.
Why other choices are wrong:
- A (PAH deficiency): Classic PKU — responds to diet restriction; phenylalanine levels would be much higher
- B (Tyrosine hydroxylase): A different enzyme in dopamine synthesis, not the regeneration step
- D (Homogentisate oxidase): Alkaptonuria — dark urine, ochronosis, arthritis — not neonatal PKU phenotype
- E (Fumarylacetoacetate hydrolase): Tyrosinemia type I — liver failure, renal Fanconi syndrome, “cabbage-like” odor
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QUESTION 2
A 25-year-old woman presents for genetic counseling after her brother was diagnosed with Huntington’s disease. She asks about her risk of developing the condition. Her father, who is currently 52 and asymptomatic, is the child of the affected individual. Genetic testing of the father reveals 36 CAG repeats in the HTT gene. What is the most important concept that should be discussed when counseling this woman about her future risk?
A) Variable expressivity due to environmental modifiers
B) Reduced penetrance at 36 repeats
C) Anticipation — repeat length may expand in subsequent generations
D) X-linked inheritance pattern with carrier status
E) Mitochondrial inheritance with maternal transmission
✅ Correct Answer: C — Anticipation
Detailed Explanation:
This question requires understanding trinucleotide repeat expansion disorders and the phenomenon of genetic anticipation.
Huntington’s Disease Basics:
- Autosomal dominant, chromosome 4 (HTT gene)
- CAG trinucleotide repeat encoding polyglutamine in the huntingtin protein
- Normal: < 26 repeats | Premutation: 27–35 | Reduced penetrance: 36–39 | Full penetrance: ≥ 40 repeats
The father has 36 repeats — in the “reduced penetrance” range. He may or may not develop HD (hence asymptomatic at 52).
Anticipation: In trinucleotide repeat disorders, the repeat number tends to expand during meiosis — particularly during paternal transmission for HD (CAG expansions occur preferentially in spermatogenesis). So the father’s 36 repeats may be transmitted to his daughter as 40, 50, or more repeats — increasing her risk of earlier onset, more severe disease.
This is the critical counseling point: her father’s 36 repeats do not define her risk — the transmitted repeat number could be significantly higher.
Trinucleotide Repeat Disorders — High-Yield Table:
| Disease | Repeat | Gene | Transmission | Anticipation |
|---|---|---|---|---|
| Huntington’s | CAG | HTT (Ch. 4) | AD; paternal expansion | Yes |
| Myotonic Dystrophy | CTG | DMPK | AD; maternal expansion | Yes |
| Fragile X | CGG | FMR1 | X-linked; maternal | Yes (maternal premutation → full mutation) |
| Friedreich’s Ataxia | GAA | Frataxin | AR | Minimal |
| SBMA (Kennedy) | CAG | Androgen receptor | X-linked recessive | Yes |
QUESTION 3
A 16-year-old boy is brought to the emergency department after collapsing during a basketball game. He is resuscitated successfully. Family history reveals that his maternal uncle died suddenly at age 22. Electrocardiogram shows a prolonged QT interval. Genetic testing reveals a loss-of-function mutation in a cardiac potassium channel gene (KCNQ1). What is the mechanism by which this mutation predisposes to fatal arrhythmia?
A) Prolonged depolarization causing triggered activity from early afterdepolarizations
B) Shortened repolarization causing re-entry circuits
C) Increased sodium influx during phase 0 causing rapid conduction
D) Decreased calcium release during phase 2 causing reduced contractility
E) Hyperkalemia-induced membrane depolarization
✅ Correct Answer: A — Prolonged depolarization causing triggered activity from early afterdepolarizations
Detailed Explanation:
This is Long QT Syndrome (LQTS) — specifically Type 1 (LQT1), caused by loss-of-function mutations in KCNQ1 (IKs potassium channel). Triggered by exercise/swimming in LQT1.
Normal Cardiac Action Potential Phases:
- Phase 0: Rapid Na⁺ influx (depolarization)
- Phase 1: K⁺ efflux (early repolarization)
- Phase 2: Ca²⁺ influx balanced with K⁺ efflux (plateau)
- Phase 3: Rapid K⁺ efflux (repolarization) — IKs (KCNQ1) is active here
- Phase 4: Resting membrane potential
How KCNQ1 loss-of-function causes LQTS: IKs channels normally drive rapid K⁺ efflux during phase 3, shortening repolarization. When these channels fail → delayed repolarization → prolonged action potential → prolonged QT interval.
During this prolonged repolarization window, spontaneous depolarizations can occur — called Early Afterdepolarizations (EADs). EADs can trigger Torsades de Pointes (a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation → sudden cardiac death).
LQTS Types to Know:
- LQT1 (KCNQ1 — IKs channel): Triggered by exercise/swimming; treat with beta-blockers, avoid swimming
- LQT2 (KCNH2/HERG — IKr channel): Triggered by sudden loud noises (alarm clocks, doorbells)
- LQT3 (SCN5A — Na⁺ channel gain-of-function): Occurs during sleep/rest; Na⁺ channels fail to inactivate → prolonged depolarization
QT-prolonging drugs to memorize (they cause acquired LQTS): “Some Antiarrhythmics Halt Conduction Favorably” — Sotalol, Amiodarone, Haloperidol, Chloroquine, Fluoroquinolones, macrolides (Azithromycin), Methadone, Ondansetron
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QUESTION 4
A newborn is found to have ambiguous genitalia. Chromosomal analysis reveals 46,XY karyotype. The infant has testes but no müllerian structures. The external genitalia appear female. Testosterone levels are elevated. What is the most likely diagnosis?
A) Congenital adrenal hyperplasia (21-hydroxylase deficiency)
B) 5-alpha reductase deficiency
C) Complete androgen insensitivity syndrome (CAIS)
D) Kallmann syndrome
E) Swyer syndrome (46,XY gonadal dysgenesis)
✅ Correct Answer: C — Complete androgen insensitivity syndrome (CAIS)
Detailed Explanation:
CAIS is a high-yield genetics/endocrinology question that tests your understanding of sexual differentiation.
Normal Sexual Differentiation Review:
- SRY gene (Y chromosome) → testes develop
- Testes produce: Testosterone (via Leydig cells) + Anti-Müllerian Hormone/AMH (via Sertoli cells)
- Testosterone → DHT (via 5-alpha reductase) → external male genitalia
- AMH → regression of Müllerian structures (uterus, fallopian tubes, upper vagina)
CAIS Mechanism: Mutation in androgen receptor (AR) gene (X-linked) → androgen receptor is completely non-functional → testosterone is produced in normal/high amounts but cannot act anywhere
Result:
- No müllerian structures (AMH works fine — not androgen-dependent) → no uterus/fallopian tubes ✓
- No virilization of external genitalia (testosterone can’t signal) → female-appearing external genitalia ✓
- Testes present (SRY works) — but located intra-abdominally or in inguinal canal ✓
- Testosterone levels elevated (no negative feedback from functional androgen receptor) ✓
- 46,XY karyotype ✓
Key Clinical Points:
- Patients present as phenotypic females, often discovered at puberty with primary amenorrhea
- Normal breast development (estrogen from testosterone aromatization works fine)
- Sparse/absent pubic and axillary hair (androgen-dependent)
- Short blind-ending vagina
- Gonads must be removed after puberty due to risk of gonadoblastoma/dysgerminoma
Why other choices are wrong:
- A (CAH, 21-hydroxylase): Causes virilization of 46,XX females (not 46,XY); cortisol/aldosterone deficiency
- B (5-alpha reductase deficiency): 46,XY with female external genitalia at birth, but undergo virilization at puberty (testosterone surges; AMH present so no müllerian structures; testosterone present but can’t convert to DHT prenatally)
- D (Kallmann): Hypogonadotropic hypogonadism + anosmia; gonads present but unstimulated
- E (Swyer syndrome): 46,XY with streak gonads (no testes), müllerian structures PRESENT (no AMH produced); phenotypically female
SECTION 2: Cardiovascular & Pulmonary (Questions 5–10)
QUESTION 5
A 45-year-old man presents with sharp chest pain that worsens when lying flat and improves when leaning forward. He had a viral upper respiratory infection 2 weeks ago. On exam, a friction rub is heard. ECG shows diffuse ST elevation with PR depression in multiple leads. What is the most dangerous immediate complication to monitor for?
A) Pulmonary embolism
B) Aortic dissection
C) Cardiac tamponade
D) Spontaneous pneumothorax
E) Unstable angina
✅ Correct Answer: C — Cardiac tamponade
Detailed Explanation:
This is acute pericarditis — the clinical picture is classic: pleuritic chest pain relieved by leaning forward, pericardial friction rub, diffuse ST elevation with PR depression (the most specific ECG finding for pericarditis, caused by atrial involvement).
The Feared Complication: Cardiac Tamponade As fluid accumulates in the pericardial sac (pericardial effusion), it can compress the heart → impaired filling → reduced cardiac output → obstructive shock.
Beck’s Triad of Cardiac Tamponade:
- Hypotension (↓ CO)
- Jugular venous distension (↑ venous backpressure — blood can’t fill heart)
- Muffled heart sounds (fluid insulates sounds)
Additional findings:
- Pulsus paradoxus: Drop in systolic BP > 10 mmHg during inspiration (pathological exaggeration of normal)
- Kussmaul’s sign: Paradoxical rise in JVD with inspiration (seen more in constrictive pericarditis)
- ECG: Electrical alternans (alternating QRS amplitude — heart swinging in fluid)
Treatment: Pericardiocentesis (needle drainage) — do NOT give diuretics (would worsen preload) or beta-blockers (compensatory tachycardia is keeping patient alive)
Pericarditis vs. MI ECG Distinction:
| Feature | Pericarditis | MI |
|---|---|---|
| ST elevation | Diffuse (multiple leads) | Localised (territory-specific) |
| PR changes | PR depression | No PR changes |
| Reciprocal changes | None | Present (ST depression in opposite leads) |
| Q waves | None | May develop |
QUESTION 6
A 68-year-old man with a long smoking history presents with progressive dyspnea and lower extremity edema. On exam, he has an elevated JVP, hepatomegaly, and a parasternal heave. His chest X-ray shows cardiomegaly with right ventricular enlargement and prominent pulmonary arteries. Echocardiogram shows elevated right ventricular pressure. Which of the following is the most likely cause of his right heart failure?
A) Left ventricular systolic dysfunction
B) Mitral stenosis
C) Chronic obstructive pulmonary disease causing cor pulmonale
D) Tricuspid valve regurgitation
E) Hypertrophic cardiomyopathy
✅ Correct Answer: C — COPD causing cor pulmonale
Detailed Explanation:
Cor pulmonale = right heart failure caused by pulmonary disease (NOT left heart disease). This is a critical distinction the NBME tests repeatedly.
Mechanism in COPD: Chronic hypoxemia and hypercapnia → hypoxic vasoconstriction of pulmonary arterioles → chronic pulmonary arterial hypertension → right ventricle must pump against elevated resistance → RV hypertrophy → eventual RV failure.
Signs of Right Heart Failure:
- Elevated JVP (blood backs up into venous system)
- Hepatomegaly (hepatic congestion)
- Peripheral edema, ascites
- Parasternal heave (RV hypertrophy palpable)
- Tricuspid regurgitation murmur (secondary to RV dilation)
Key Distinction: In cor pulmonale, the LEFT ventricle is normal. In biventricular failure from LV dysfunction (choice A), the right heart fails secondarily to left heart failure — and you’d expect pulmonary edema and elevated pulmonary capillary wedge pressure.
Causes of Cor Pulmonale:
- COPD (most common)
- Pulmonary fibrosis (IPF)
- Obstructive sleep apnea
- Recurrent pulmonary emboli
- Primary pulmonary arterial hypertension
QUESTION 7
A 32-year-old woman presents with sudden onset dyspnea, pleuritic chest pain, and mild hemoptysis 10 days after a long-haul flight from Tokyo. She is on oral contraceptives. CT pulmonary angiography confirms a pulmonary embolism. Which of the following best explains the pathophysiology of hemoptysis in this patient?
A) Erosion of the bronchial mucosa by the embolus
B) Pulmonary infarction causing hemorrhagic necrosis of lung parenchyma
C) Reperfusion injury after spontaneous clot dissolution
D) Coagulopathy from factor consumption
E) Pulmonary arteriovenous fistula formation
✅ Correct Answer: B — Pulmonary infarction causing hemorrhagic necrosis of lung parenchyma
Detailed Explanation:
The lung has a dual blood supply: pulmonary arteries AND bronchial arteries. Because of this redundancy, most pulmonary emboli do NOT cause pulmonary infarction — the bronchial artery can maintain oxygenation to the affected parenchyma.
However, pulmonary infarction (and therefore hemoptysis) occurs when:
- The patient has underlying cardiopulmonary disease (heart failure, prior lung disease) that has compromised bronchial artery flow, OR
- The embolus is peripheral and small (lodging in a distal branch where collateral flow is less robust)
When infarction does occur → hemorrhagic (red) infarction because the dual blood supply means blood continues to seep into the infarcted zone from the bronchial circulation → bloody, wedge-shaped infarct pointing toward the hilum (Hampton’s hump on CXR).
Classic CXR findings in PE:
- Hampton’s hump: Wedge-shaped peripheral opacity (infarction)
- Westermark sign: Focal oligemia (decreased vascular markings distal to occlusion)
- Most commonly: Normal CXR (especially early)
- Atelectasis, small pleural effusion
This patient’s risk factors: OCP (estrogen increases clotting factors, decreases protein S) + prolonged immobility (long-haul flight) + Virchow’s triad (stasis + hypercoagulability) = classic PE setup
QUESTION 8
A 4-year-old boy is noted to have a loud, harsh holosystolic murmur at the left sternal border since birth. He has a history of frequent respiratory infections. Echocardiogram shows a ventricular septal defect with a left-to-right shunt. If left untreated for many years, which of the following would represent the most serious irreversible complication?
A) Progression to complete heart block
B) Reversal of shunt direction with cyanosis (Eisenmenger syndrome)
C) Development of aortic stenosis from turbulent flow
D) Complete spontaneous closure of the defect
E) Mitral valve prolapse from ventricular remodeling
✅ Correct Answer: B — Reversal of shunt direction with cyanosis (Eisenmenger syndrome)
Detailed Explanation:
Eisenmenger Syndrome is one of the most elegant — and tragic — concepts in congenital heart disease, and the NBME loves it.
The Progression:
- VSD with left-to-right shunt: Oxygenated blood from LV flows into RV → increased pulmonary blood flow
- Chronic pulmonary overcirculation → pulmonary vascular remodeling → pulmonary hypertension
- As pulmonary arterial pressure rises, it eventually exceeds systemic pressure → shunt reverses → right-to-left shunt
- Deoxygenated blood now flows from RV → LV → systemic circulation → late-onset cyanosis (differential cyanosis in PDA — lower body cyanotic; upper body not)
- At this point, the defect cannot be surgically closed — closing it would acutely remove the “pop-off” mechanism and cause acute RV failure
Why it’s irreversible: The pulmonary vascular remodeling (intimal proliferation, medial hypertrophy, plexiform lesions) is permanent. Closing the VSD at this stage doesn’t reverse the pulmonary hypertension.
Clinical Features of Eisenmenger:
- Cyanosis (late onset, years after birth)
- Clubbing of fingers
- Polycythemia (compensatory, from chronic hypoxia)
- Paradoxical emboli (venous thrombi can cross to arterial circulation)
- Hemoptysis
Eisenmenger can complicate any large, uncorrected left-to-right shunt: VSD, ASD, PDA
SECTION 3: Microbiology & Immunology (Questions 9–14)
QUESTION 9
A 35-year-old man with HIV (CD4 count 45 cells/µL) presents with 3 weeks of progressive headache, low-grade fever, and altered mental status. Lumbar puncture shows: opening pressure 280 mmHg, clear CSF, WBC 8 cells/µL (predominantly lymphocytes), protein mildly elevated, glucose normal. India ink stain of CSF reveals encapsulated organisms. What virulence factor of this organism most directly helps it evade the immune system?
A) Protein A binding to IgG Fc region
B) IgA protease cleaving mucosal antibodies
C) Polysaccharide capsule inhibiting phagocytosis
D) Coagulase converting fibrinogen to fibrin
E) Beta-lactamase hydrolyzing antibiotic cell wall agents
✅ Correct Answer: C — Polysaccharide capsule inhibiting phagocytosis
Detailed Explanation:
This is Cryptococcal meningitis — the most common cause of fungal meningitis, occurring almost exclusively in severely immunocompromised patients (CD4 < 100 cells/µL in HIV).
Organism: Cryptococcus neoformans — found in pigeon droppings, soil. Inhaled → disseminates hematogenously → CNS.
Diagnosis:
- India ink stain: Encapsulated yeast with clear halo (capsule repels ink)
- Cryptococcal antigen (CrAg): Highly sensitive, detects capsular polysaccharide
- CSF: Elevated opening pressure (can cause vision loss from increased ICP), lymphocytic pleocytosis, normal glucose
The Capsule’s Role: The polysaccharide capsule of C. neoformans is its primary virulence factor:
- Inhibits phagocytosis by alveolar macrophages and neutrophils (can’t engulf organism)
- Impairs complement activation (C3b opsonization is blocked)
- Inhibits leukocyte migration into CSF
- Dysregulates cytokine release — immunosuppressive environment favoring fungal survival
Treatment: Induction with Amphotericin B + Flucytosine (2 weeks) → consolidation with Fluconazole (8 weeks) → maintenance fluconazole lifelong until CD4 > 200
Why other choices are wrong:
- A (Protein A): S. aureus virulence factor — binds Fc of IgG, blocking opsonization
- B (IgA protease): S. pneumoniae, N. meningitidis, H. influenzae — colonize mucosal surfaces
- D (Coagulase): S. aureus — converts fibrinogen to fibrin, walling off infection
- E (Beta-lactamase): Antibiotic resistance mechanism (bacteria) — not an immune evasion factor
QUESTION 10
A 19-year-old college student presents with fever, severe headache, nuchal rigidity, and a petechial rash spreading rapidly over his trunk and lower extremities. He was found unresponsive in his dormitory. Blood cultures are drawn and he is immediately treated. The organism responsible is also a leading cause of bacterial meningitis in his age group. Which of the following best describes the mechanism by which this organism’s cell wall component triggers septic shock?
A) Peptidoglycan activating NOD receptors in macrophages
B) Teichoic acid binding to TLR-2 on innate immune cells
C) Lipopolysaccharide (LPS) binding to TLR-4 via CD14, activating NF-κB
D) Exotoxin A inhibiting EF-2 and halting protein synthesis
E) Superantigen cross-linking MHC II with T cell receptor
✅ Correct Answer: C — LPS binding to TLR-4 via CD14, activating NF-κB
Detailed Explanation:
This is Neisseria meningitidis meningitis — the most feared cause of bacterial meningitis in teenagers and young adults (especially college dormitories). The petechial/purpuric rash from Waterhouse-Friderichsen syndrome (bilateral adrenal hemorrhage from DIC) is the clinical giveaway.
N. meningitidis is gram-negative → has lipopolysaccharide (LPS/endotoxin) in its outer membrane.
LPS-Mediated Septic Shock Pathway:
- LPS released from lysed bacteria
- LPS-binding protein (LBP) in serum binds LPS → delivers to CD14 on macrophage surface
- CD14 presents LPS to TLR-4 (Toll-Like Receptor 4)
- TLR-4 activation → MyD88 signaling → NF-κB transcription factor activated
- NF-κB → massive transcription of pro-inflammatory cytokines: TNF-α, IL-1, IL-6, IL-12
- TNF-α → vasodilation, endothelial damage, DIC, multi-organ failure → septic shock
Treatment of N. meningitidis:
- Ceftriaxone (immediate — do NOT delay for LP if clinical suspicion is high)
- Dexamethasone (reduces inflammation, improves outcomes)
- Rifampin or ciprofloxacin for close contact prophylaxis
- Vaccine: MCV4 (meningococcal conjugate vaccine) — recommended for adolescents
QUESTION 11
A 6-year-old boy presents with honey-crusted lesions around his nose and mouth. Gram stain of the lesion exudate shows gram-positive cocci in clusters. Culture grows an organism that is catalase-positive and coagulase-positive. Which toxin produced by this organism acts by cleaving desmoglein-1?
A) Alpha toxin
B) Toxic shock syndrome toxin-1 (TSST-1)
C) Exfoliative toxin (ET)
D) Panton-Valentine Leukocidin (PVL)
E) Protein A
✅ Correct Answer: C — Exfoliative toxin (ET)
Detailed Explanation:
Organism: Staphylococcus aureus — gram-positive cocci in clusters, catalase-positive, coagulase-positive (distinguishes S. aureus from coagulase-negative staph like S. epidermidis).
The honey-crusted lesions are impetigo — but this question is asking about a specific toxin mechanism.
Exfoliative Toxin (ET):
- Produced by certain strains of S. aureus
- Acts as a serine protease that cleaves desmoglein-1 — a desmosomal cadherin protein that holds keratinocytes together in the stratum granulosum
- Disrupting desmoglein-1 → separation of skin layers → bullous impetigo (local ET) or Staphylococcal Scalded Skin Syndrome (SSSS) (systemic ET, from bacteremia)
SSSS vs. Toxic Epidermal Necrolysis (TEN):
| Feature | SSSS | TEN |
|---|---|---|
| Cause | S. aureus ET | Drug reaction (sulfonamides, anticonvulsants) |
| Age | Children < 5 (lack antibodies to ET) | Adults |
| Split level | Stratum granulosum (superficial) | Dermal-epidermal junction (deep) |
| Mucous membranes | Spared | Involved |
| Nikolsky sign | Positive | Positive |
| Mortality | Low (< 5% in children) | High (25–35%) |
S. aureus Toxin Summary:
- Alpha toxin: Pore-forming toxin → cell lysis (RBCs, WBCs)
- TSST-1: Superantigen → Toxic Shock Syndrome (fever, rash, hypotension, multi-organ failure); associated with tampon use
- Exfoliative toxin: Cleaves desmoglein-1 → skin separation (SSSS, bullous impetigo)
- PVL (Panton-Valentine Leukocidin): Pore-forming toxin → kills WBCs; associated with skin/soft tissue infections and necrotizing pneumonia in community-acquired MRSA
QUESTION 12
A 45-year-old HIV-positive man (CD4 count 160 cells/µL) presents with a 2-cm painless ulcer on his penis with a clean, indurated base and rubbery, non-tender inguinal lymphadenopathy. He reports a new sexual partner 3 weeks ago. Dark-field microscopy of the ulcer exudate shows motile spiral-shaped organisms. RPR is positive. What is the treatment of choice?
A) Azithromycin 1g orally, single dose
B) Ceftriaxone 250 mg IM, single dose
C) Benzathine penicillin G 2.4 million units IM, single dose
D) Doxycycline 100 mg BID for 14 days
E) Acyclovir 400 mg TID for 7 days
✅ Correct Answer: C — Benzathine penicillin G 2.4 million units IM, single dose
Detailed Explanation:
This is primary syphilis — caused by Treponema pallidum (spirochete, visualized on dark-field microscopy). The painless indurated ulcer (chancre) + non-tender lymphadenopathy is the classic presentation of the primary stage.
Why Benzathine Penicillin G? T. pallidum has remained exquisitely sensitive to penicillin for over 70 years — no resistance has developed. Benzathine penicillin G provides sustained low-level penicillin levels for weeks, which is necessary because T. pallidum divides very slowly (every 30–33 hours) and requires prolonged antibiotic exposure.
Treatment by Stage:
- Primary, secondary, early latent (< 1 year): Benzathine penicillin G 2.4 million units IM × 1 dose
- Late latent or unknown duration: Benzathine penicillin G 2.4 million units IM weekly × 3 doses
- Neurosyphilis: Aqueous crystalline penicillin G IV × 10–14 days (benzathine doesn’t penetrate CNS adequately)
- Penicillin allergy (non-pregnant): Doxycycline 100 mg BID × 14 days
- Penicillin allergy (pregnant): Desensitize and give penicillin — no alternative is safe in pregnancy
Jarisch-Herxheimer Reaction: Fever, chills, headache, myalgias within 24 hours of treatment — caused by massive spirochete lysis releasing cytokines. Treat with antipyretics; do NOT stop antibiotics.
Syphilis Stages:
- Primary: Painless chancre (3 weeks after exposure) → heals spontaneously
- Secondary: Disseminated — maculopapular rash on palms and soles, condyloma lata, mucous patches, systemic symptoms (weeks to months later)
- Latent: Asymptomatic; early (<1 year) vs. late (>1 year)
- Tertiary: Gummas, cardiovascular syphilis (aortitis → aortic aneurysm), neurosyphilis (tabes dorsalis, Argyll Robertson pupil)
SECTION 4: Neuroscience & Psychiatry (Questions 13–18)
QUESTION 13
A 72-year-old man is brought by his family for memory loss over the past 2 years. He has difficulty remembering recent events but recalls distant memories well. He often gets confused in the evening (“sundowning”). Mini-Mental State Examination score is 19/30. MRI shows cortical atrophy predominantly in the temporal and parietal lobes and hippocampal atrophy. What is the most likely neuropathological finding?
A) Lewy bodies in the substantia nigra and cortex
B) Tau-positive neurofibrillary tangles and beta-amyloid plaques
C) Prion protein deposits causing spongiform changes
D) Intranuclear inclusions in striatal neurons
E) TDP-43 positive inclusions in motor neurons
✅ Correct Answer: B — Tau-positive neurofibrillary tangles and beta-amyloid plaques
Detailed Explanation:
This is Alzheimer’s Disease (AD) — the most common cause of dementia in the elderly, characterized by insidious onset, progressive anterograde amnesia (can’t form new memories), and relatively preserved remote memory early on.
Neuropathology of AD — Two Major Findings:
1. Beta-amyloid plaques (neuritic/senile plaques):
- Located extracellularly
- Composed of Aβ-42 peptide (derived from amyloid precursor protein/APP by beta- and gamma-secretase cleavage)
- Aβ aggregates → oligomers → plaques → neurotoxic
- Associated with Down syndrome (Trisomy 21) — APP gene on chromosome 21; nearly all DS patients develop AD by age 40
2. Neurofibrillary tangles (NFTs):
- Located intracellularly in neurons
- Composed of hyperphosphorylated tau protein — normally tau stabilizes microtubules; when hyperphosphorylated, it dissociates and aggregates into tangles → microtubule collapse → neuronal dysfunction and death
Regions most affected: Entorhinal cortex → hippocampus → temporal/parietal cortex → eventually frontal cortex
Neurotransmitter Deficit: Loss of cholinergic neurons in nucleus basalis of Meynert → decreased ACh → memory impairment. Treatment: AChE inhibitors (donepezil, rivastigmine, galantamine) + memantine (NMDA antagonist) for moderate-severe AD.
Dementia Differential:
| Feature | Alzheimer’s | Lewy Body | Frontotemporal | Vascular |
|---|---|---|---|---|
| Memory | Early | Moderate | Late (behavior first) | Variable |
| Hallucinations | Late | Early (visual) | Behavioral changes | Stepwise decline |
| Path | Plaques + NFTs | Lewy bodies (alpha-synuclein) | TDP-43/tau inclusions | Lacunar infarcts |
| Key feature | Sundowning, anterograde amnesia | Parkinsonism, REM sleep disorder | Disinhibition, apathy | Stroke history, focal deficits |
QUESTION 14
A 28-year-old woman presents after a first witnessed seizure. She describes a feeling of “déjà vu” and an unusual smell before losing awareness briefly. She had no convulsions. EEG shows focal epileptiform discharges in the temporal region. MRI shows hippocampal sclerosis. What type of seizure does this represent?
A) Absence seizure
B) Tonic-clonic seizure
C) Focal seizure with impaired awareness (complex partial seizure)
D) Myoclonic seizure
E) Atonic seizure
✅ Correct Answer: C — Focal seizure with impaired awareness
Detailed Explanation:
Seizure Classification (2017 ILAE):
Focal seizures (start in one hemisphere):
- Focal aware (simple partial, old terminology): Consciousness preserved; aura-like symptoms (déjà vu, olfactory hallucinations, automatisms)
- Focal with impaired awareness (complex partial): Consciousness is altered; patient appears confused, staring, with automatisms
- Focal → bilateral tonic-clonic: Secondary generalization
This patient’s seizure features:
- Aura: Déjà vu + olfactory hallucination (“unusual smell”) → temporal lobe onset (temporal lobe processes memory, emotion, smell)
- Loss of awareness → “focal with impaired awareness”
- No convulsions → not tonic-clonic
- EEG: Temporal focus
- MRI: Hippocampal sclerosis — most common cause of temporal lobe epilepsy
Treatment: First-line for focal epilepsy — levetiracetam, carbamazepine, lamotrigine, oxcarbazepine
Distinguishing Absence from Complex Partial:
| Feature | Absence (Petit Mal) | Focal with Impaired Awareness |
|---|---|---|
| Age | Children | Any age |
| Duration | 5–20 seconds | 1–3 minutes |
| Aura | None | Common |
| Post-ictal confusion | None | Yes |
| EEG | 3 Hz spike-wave | Temporal/focal spike |
| Treatment | Ethosuximide, valproate | Levetiracetam, carbamazepine |
QUESTION 15
A 35-year-old man presents with 6 months of progressive weakness in his hands, difficulty swallowing, and dysarthria. On exam, he has both upper motor neuron signs (hyperreflexia, spasticity, Babinski sign) AND lower motor neuron signs (muscle atrophy, fasciculations) at the same level. Sensation is intact. What is the most likely diagnosis?
A) Multiple sclerosis
B) Syringomyelia
C) Amyotrophic lateral sclerosis (ALS)
D) Guillain-Barré syndrome
E) Brown-Séquard syndrome
✅ Correct Answer: C — Amyotrophic lateral sclerosis (ALS)
Detailed Explanation:
ALS is the textbook diagnosis when you see both UMN and LMN signs at the same anatomical level with preserved sensation.
The Pathological Key: ALS selectively destroys motor neurons only — upper (cortex/corticospinal tract) AND lower (anterior horn cells/motor cranial nerve nuclei). Sensory pathways and cognition are spared (in most cases).
UMN signs: Hyperreflexia, spasticity, Babinski sign (upgoing plantar reflex), clonus
LMN signs: Flaccid weakness, muscle atrophy, fasciculations, hyporeflexia
Why the distractors are wrong:
- A (MS): Demyelinating disease affecting white matter; causes UMN signs + sensory changes + cranial nerve involvement; NO LMN signs; relapsing-remitting course
- B (Syringomyelia): Cavity in spinal cord center → initially destroys crossing spinothalamic fibers (pain/temp loss) → “cape-like” sensory loss; eventually LMN signs; NO UMN signs early
- D (GBS): Purely LMN (peripheral neuropathy); ascending weakness, areflexia, albuminocytologic dissociation on LP; associated with Campylobacter or viral illness
- E (Brown-Séquard): Hemisection of spinal cord → ipsilateral UMN + ipsilateral loss of proprioception + contralateral loss of pain/temp; sensory involved
ALS Prognosis: Median survival 3–5 years from symptom onset; bulbar onset (dysarthria, dysphagia) has worse prognosis. Riluzole (glutamate antagonist) and Edaravone are the only FDA-approved treatments — modest effect on survival.
SECTION 5: Endocrinology & Reproductive (Questions 16–20)
QUESTION 16
A 45-year-old woman presents with a 6-month history of heat intolerance, weight loss despite increased appetite, palpitations, and diarrhea. On exam, she has a diffuse, non-tender goiter, exophthalmos, and pretibial myxedema. TSH is undetectable; free T4 is markedly elevated. What antibody is most responsible for this condition?
A) Anti-thyroid peroxidase (anti-TPO) antibodies
B) Anti-thyroglobulin antibodies
C) TSH receptor-stimulating antibodies (TRAb)
D) Anti-TSH antibodies
E) Anti-T3 antibodies
✅ Correct Answer: C — TSH receptor-stimulating antibodies (TRAb)
Detailed Explanation:
This is Graves’ Disease — the most common cause of hyperthyroidism in young to middle-aged women, and an autoimmune condition.
Pathophysiology: IgG autoantibodies (TRAb, also called TSI — thyroid-stimulating immunoglobulins) bind the TSH receptor on thyroid follicular cells → mimic TSH → continuous stimulation → thyroid hormone hypersecretion → hyperthyroidism.
Because the antibody continuously stimulates the receptor regardless of T4 levels, normal negative feedback is bypassed — pituitary cannot suppress thyroid production → undetectable TSH, high T4/T3.
The Unique Clinical Features of Graves’ (not seen in other hyperthyroidism causes):
- Exophthalmos (proptosis): TRAb stimulates orbital fibroblasts → glycosaminoglycan deposition → orbital swelling → eye protrusion. Can cause corneal ulceration if severe.
- Pretibial myxedema: Skin thickening over shins — glycosaminoglycan deposits (confusingly, NOT related to hypothyroidism in this context)
- Thyroid bruit: Increased vascularity of goiter
Treatment Options:
- Methimazole (first-line medical) — inhibits thyroid peroxidase, blocks hormone synthesis; teratogenic in 1st trimester
- Propylthiouracil (PTU) — preferred in 1st trimester pregnancy (methimazole causes choanal atresia); also blocks peripheral T4→T3 conversion
- Radioactive iodine (RAI/I-131) — destroys thyroid tissue; preferred definitive treatment in most non-pregnant adults; contraindicated in pregnancy
- Thyroidectomy — for large goiters, RAI failure, or pregnancy with inadequate drug control
- Beta-blockers (propranolol) — symptomatic relief (palpitations, tremor, heat intolerance); propranolol also blocks T4→T3 conversion
QUESTION 17
A 58-year-old man with Type 2 diabetes presents for a routine check. His HbA1c is 8.9%. He also has a BMI of 38 kg/m² and a GFR of 55 mL/min/1.73m². His physician adds a medication that works by blocking glucose reabsorption in the proximal tubule, leading to glucosuria. Which of the following is the most important additional benefit of this drug class beyond glycemic control?
A) Reduces hepatic glucose production
B) Reduces cardiovascular mortality and slows progression of diabetic kidney disease
C) Stimulates insulin secretion from pancreatic beta cells
D) Promotes weight gain through increased caloric retention
E) Lowers triglycerides by activating PPAR-alpha
✅ Correct Answer: B — Reduces cardiovascular mortality and slows progression of diabetic kidney disease
Detailed Explanation:
This is an SGLT-2 inhibitor (empagliflozin, dapagliflozin, canagliflozin) question — one of the most important drug classes in modern diabetes management and a heavily tested Step 2 CK / Step 1 pharmacology topic.
Mechanism: SGLT-2 (sodium-glucose cotransporter 2) is responsible for ~90% of glucose reabsorption in the proximal convoluted tubule. Blocking it → glucosuria → osmotic diuresis → modest glycemic lowering.
The Landmark Benefits Beyond Glucose Control: Multiple large cardiovascular outcomes trials (EMPA-REG, CREDENCE, DAPA-CKD) demonstrated that SGLT-2 inhibitors:
- Reduce cardiovascular death and heart failure hospitalizations in patients with established CVD or high CV risk
- Slow progression of diabetic nephropathy and reduce risk of ESRD (through hemodynamic effects on glomerular pressure — similar mechanism to ACE inhibitors)
- Reduce body weight (glucosuria = caloric loss) — opposite of choice D
- Reduce blood pressure (osmotic diuresis)
These cardiorenal benefits are now the primary reason SGLT-2 inhibitors are recommended in T2DM patients with heart failure or CKD, even independent of glycemic status.
Important Adverse Effects:
- Genitourinary infections (glucosuria creates favorable environment for Candida and bacterial UTIs)
- Euglycemic DKA (rare but dangerous — glucose may be near-normal; check in perioperative settings)
- Fournier’s gangrene (necrotizing fasciitis of perineum — black box warning)
- Bone fractures, volume depletion (canagliflozin)
- Amputation risk (canagliflozin — black box warning)
SECTION 6: Hematology & Oncology (Questions 18–22)
QUESTION 18
A 24-year-old African American man presents with fatigue, jaundice, and dark urine. He was recently started on primaquine. Labs: Hgb 7.8 g/dL, MCV 85 fL, LDH elevated, indirect bilirubin elevated, haptoglobin low. Peripheral smear shows bite cells. Direct Coombs test is negative. Which finding on hemoglobin electrophoresis would be expected in this patient?
A) HbS predominance with absent HbA
B) HbC with target cells on smear
C) Normal electrophoresis — G6PD deficiency is not detected by this test
D) HbH (4 beta chains) indicating alpha-thalassemia
E) Elevated HbA2 and HbF indicating beta-thalassemia trait
✅ Correct Answer: C — Normal electrophoresis — G6PD deficiency is not detected by this test
Detailed Explanation:
This is G6PD deficiency — triggered by primaquine (antimalarial), presenting with hemolytic anemia. The question is asking about the diagnostic test.
Critical point: G6PD deficiency is an enzyme deficiency — not a hemoglobin structural abnormality. Hemoglobin electrophoresis separates hemoglobin variants by charge (detects HbS, HbC, HbA2, etc.) but has no ability to detect enzyme deficiencies. The electrophoresis will be completely normal.
Correct Diagnostic Tests for G6PD Deficiency:
- G6PD enzyme activity assay (fluorescent spot test or quantitative G6PD assay)
- Important caveat: Test immediately BEFORE or well AFTER a hemolytic episode — during acute hemolysis, G6PD-deficient cells have already been destroyed, leaving only the youngest reticulocytes (which have the highest G6PD activity) → the test may appear falsely normal during acute crisis
Smear findings in G6PD:
- Bite cells (eccentrocytes) — spleen removes Heinz bodies, taking a “bite” of cytoplasm
- Heinz bodies — intracellular precipitates of denatured hemoglobin (seen with crystal violet stain, not routine H&E)
- Blister cells
QUESTION 19
A 68-year-old man is found to have a WBC of 95,000/µL on routine CBC. He is completely asymptomatic. Flow cytometry of peripheral blood shows a monoclonal population of mature lymphocytes positive for CD5, CD19, CD20 (dim), and CD23, and negative for FMC7 and CD10. Peripheral smear shows numerous small, mature lymphocytes and “smudge cells.” What is the most likely diagnosis?
A) Acute lymphoblastic leukemia (ALL)
B) Mantle cell lymphoma
C) Follicular lymphoma
D) Chronic lymphocytic leukemia (CLL)
E) Hairy cell leukemia
✅ Correct Answer: D — Chronic lymphocytic leukemia (CLL)
Detailed Explanation:
The immunophenotype is the diagnostic key here — this is a flow cytometry question.
CLL Immunophenotype:
- CD5+ (T cell marker co-expressed on CLL cells — the hallmark anomaly)
- CD19+, CD20 (dim)+, CD23+ — B cell markers
- FMC7 negative, CD10 negative
- Low-level surface immunoglobulin
CD5+ B cell neoplasms (memorize just two):
- CLL (CD23+, FMC7-)
- Mantle cell lymphoma (CD23-, FMC7+, cyclin D1+, t[11;14])
CLL vs. Mantle Cell:
| Feature | CLL | Mantle Cell Lymphoma |
|---|---|---|
| CD5 | ✅ Positive | ✅ Positive |
| CD23 | Positive | Negative |
| FMC7 | Negative | Positive |
| Cyclin D1 | Negative | Positive |
| Translocation | — | t(11;14) |
| Prognosis | Indolent | Aggressive |
CLL High-Yield Pearls:
- Most common leukemia in adults in Western countries
- “Smudge cells” on smear — CLL cells are fragile and smear during slide preparation → pathognomonic
- Most patients are asymptomatic at diagnosis (“watch and wait”)
- Richter transformation: CLL → aggressive diffuse large B cell lymphoma (sudden lymphadenopathy, weight loss, fever)
- Warm AIHA commonly complicates CLL (IgG antibodies vs. RBCs → Coombs positive)
- Hypogammaglobulinemia → recurrent bacterial infections
QUESTION 20
A 35-year-old woman presents with a 3-cm painless neck mass. Fine-needle aspiration shows cells with ground-glass nuclei (“Orphan Annie eye” nuclei), nuclear grooves, and psammoma bodies. Which of the following molecular alterations is most commonly associated with this tumor?
A) RET/PTC rearrangement or BRAF V600E mutation
B) RAS mutation with PAX8-PPARγ rearrangement
C) RET point mutation (germline)
D) PTEN loss with follicular architecture
E) p53 mutation with anaplastic transformation
✅ Correct Answer: A — RET/PTC rearrangement or BRAF V600E mutation
Detailed Explanation:
Papillary Thyroid Carcinoma (PTC) — the most common thyroid malignancy, and one of the most tested oncology topics on Step 1.
Histological Features of PTC:
- Ground-glass/Orphan Annie eye nuclei — clear nuclei with centrally placed nucleolus
- Nuclear grooves — coffee-bean appearance
- Psammoma bodies — concentric calcified deposits (“Pap smear, Papillary, Psammoma” — also in serous ovarian carcinoma, meningioma)
- Finger-like papillary projections
Molecular Alterations:
- BRAF V600E mutation (~60% of cases) — most common; associated with aggressive behavior; targetable with vemurafenib
- RET/PTC rearrangement — fusion oncogene from chromosomal rearrangement; associated with radiation exposure (post-Chernobyl children)
Thyroid Cancer Molecular Associations:
| Cancer Type | Molecular Marker | Key Features |
|---|---|---|
| Papillary (most common) | BRAF V600E, RET/PTC | Ground-glass nuclei, psammoma bodies, spreads to lymph nodes, excellent prognosis |
| Follicular | RAS, PAX8-PPARγ | Follicular architecture, NO psammoma bodies, hematogenous spread (bone, lung) |
| Medullary | RET point mutation (germline in MEN 2A/2B) | C-cells → calcitonin; amyloid in stroma; associated with MEN syndromes |
| Anaplastic | p53 mutation | Elderly, rapidly fatal, no differentiation |
SECTION 7: Reproductive, GI & Musculoskeletal (Questions 21–25)
QUESTION 21
A 27-year-old woman at 10 weeks gestation presents with severe nausea, vomiting, and vaginal bleeding. Uterine size is large for gestational age. Serum β-hCG is 220,000 mIU/mL (markedly elevated). Ultrasound shows a uterus filled with a “snowstorm” pattern and no fetal parts. What is the complication most important to monitor for after treatment?
A) Ectopic pregnancy in the opposite fallopian tube
B) Invasive mole or choriocarcinoma (gestational trophoblastic neoplasia)
C) Placenta previa in future pregnancies
D) Cervical incompetence
E) Recurrent miscarriage from antiphospholipid syndrome
✅ Correct Answer: B — Invasive mole or choriocarcinoma (gestational trophoblastic neoplasia)
Detailed Explanation:
This is a complete hydatidiform mole — abnormal proliferation of trophoblastic tissue with no fetal development.
Complete Mole:
- 46,XX karyotype — all paternal (2 sperm fertilize empty egg, or 1 sperm duplicates)
- NO fetal parts
- “Snowstorm” on ultrasound (grape-like clusters of hydropic villi)
- Markedly elevated β-hCG (trophoblasts produce β-hCG in massive amounts)
- Presents with vaginal bleeding, hyperemesis gravidarum, early pre-eclampsia (<20 weeks)
Treatment: Suction curettage → send tissue to pathology
Critical Post-Treatment Monitoring: After mole evacuation, serial β-hCG levels must be followed weekly until undetectable. Reason: ~15–20% of complete moles progress to Gestational Trophoblastic Neoplasia (GTN):
- Invasive mole (locally invasive into myometrium)
- Choriocarcinoma (highly malignant; metastasizes to lungs, brain; extremely sensitive to chemotherapy — one of the most curable solid tumors even when metastatic)
Rising or plateauing β-hCG after mole → GTN → chemotherapy (methotrexate or actinomycin-D)
Patients are advised to avoid pregnancy for 6–12 months to allow accurate β-hCG monitoring.
QUESTION 22
A 52-year-old man presents with a 4-month history of dysphagia, initially to solids and now to both solids and liquids, along with an unintentional 15-pound weight loss. He has a 25-pack-year smoking history and drinks 3–4 alcoholic drinks per day. Endoscopy shows a mass in the middle third of the esophagus. Biopsy reveals squamous cell carcinoma. Which area of the esophagus is at highest risk for this tumor?
A) Lower esophagus adjacent to the gastroesophageal junction
B) Upper esophagus at the cricopharyngeus
C) Middle third of the esophagus
D) Distal esophagus with Barrett’s changes
E) Cervical esophagus
✅ Correct Answer: C — Middle third of the esophagus
Detailed Explanation:
Esophageal Cancer — Two Types, Two Locations:
| Feature | Squamous Cell Carcinoma | Adenocarcinoma |
|---|---|---|
| Location | Middle third | Lower third / GEJ |
| Risk factors | Alcohol + tobacco (biggest RF), achalasia, caustic ingestion, Plummer-Vinson | GERD, Barrett’s esophagus, obesity |
| Histology | Squamous cells, keratin pearls | Glandular cells |
| Demographics | More common in Black men, globally | More common in White men, rising incidence in US |
| Worldwide | More common globally | More common in USA |
This patient: Alcohol + tobacco + middle-third location → squamous cell carcinoma ✓
Barrett’s Esophagus → Adenocarcinoma Pathway: Chronic GERD → chronic acid exposure → metaplasia of squamous epithelium to columnar (intestinal-type) epithelium → Barrett’s esophagus → dysplasia → adenocarcinoma.
Endoscopic surveillance with biopsy is recommended in Barrett’s patients to detect dysplasia early.
Progressive Dysphagia Pattern:
- Solids first, then liquids = mechanical obstruction (tumor, stricture)
- Both solids and liquids simultaneously = motility disorder (achalasia, diffuse esophageal spasm)
QUESTION 23
A 55-year-old woman presents with symmetric pain and swelling of the bilateral proximal interphalangeal (PIP) and metacarpophalangeal (MCP) joints, morning stiffness lasting over 2 hours, and fatigue for 8 months. Labs show elevated CRP and ESR, positive anti-CCP antibodies, and rheumatoid factor positive. X-ray shows periarticular osteopenia and marginal erosions. What is the pathophysiology of joint destruction in this condition?
A) Uric acid crystal deposition causing neutrophilic inflammation
B) Calcium pyrophosphate deposition in fibrocartilage
C) Synovial T cell and macrophage activation with pannus formation
D) Septic bacteria destroying cartilage by direct invasion
E) Anti-DNA antibodies forming immune complexes in synovial tissue
✅ Correct Answer: C — Synovial T cell and macrophage activation with pannus formation
Detailed Explanation:
This is Rheumatoid Arthritis (RA) — and the question is testing the mechanism of joint destruction.
RA Pathophysiology:
- Unknown trigger (infection? smoking?) activates CD4+ Th1 and Th17 T cells in the synovium
- T cells activate macrophages and B cells → B cells produce rheumatoid factor (anti-IgG Fc) and anti-CCP antibodies (most specific for RA)
- Macrophages and T cells release TNF-α, IL-1, IL-6, IL-17 → synovial inflammation
- Synovial cells proliferate → form pannus (destructive granulation tissue that invades cartilage and bone)
- Pannus produces proteases and RANKL → activates osteoclasts → marginal bone erosions (seen on X-ray)
Pathological Sequence: Synovitis → Pannus formation → Cartilage erosion → Bone erosion → Joint deformity
Treatment Targets:
- Methotrexate (first-line DMARD — folate antagonist, anti-inflammatory)
- TNF-α inhibitors (etanercept, infliximab, adalimumab) — biologics for refractory disease
- IL-6 inhibitor (tocilizumab)
- JAK inhibitors (tofacitinib) — oral targeted DMARDs
- NSAIDs + steroids for symptomatic relief only
SECTION 8: Statistics, Epidemiology & Ethics (Questions 24–25)
QUESTION 24
A study evaluates a new blood test to screen for colon cancer. The test is performed on 1,000 individuals — 100 of whom have colon cancer confirmed by colonoscopy. The test correctly identifies 85 of the 100 cancer patients as positive, and correctly identifies 810 of the 900 cancer-free individuals as negative. What is the specificity of this test?
A) 81%
B) 85%
C) 89%
D) 90%
E) 93%
✅ Correct Answer: D — 90%
Detailed Explanation:
Statistics questions appear on every USMLE exam. This is a 2×2 table question — the most common format.
Building the 2×2 Table:
| Disease + | Disease – | Total | |
|---|---|---|---|
| Test + | 85 (TP) | 90 (FP) | 175 |
| Test – | 15 (FN) | 810 (TN) | 825 |
| Total | 100 | 900 | 1000 |
Formulas:
- Sensitivity = TP / (TP + FN) = 85 / 100 = 85% (how well the test detects true positives)
- Specificity = TN / (TN + FP) = 810 / 900 = 90% (how well the test rules out disease in healthy people)
- PPV = TP / (TP + FP) = 85 / 175 = 48.6% (probability of disease given positive test)
- NPV = TN / (TN + FN) = 810 / 825 = 98.2% (probability of no disease given negative test)
The Critical USMLE Relationships:
- Sensitivity is used for screening tests (don’t want to miss disease — high sensitivity means few false negatives)
- Specificity is used for confirmatory tests (want to be sure before diagnosing — high specificity means few false positives)
- “SnNout” — high Sensitivity test, Negative result = rules OUT disease
- “SpPin” — high Specificity test, Positive result = rules IN disease
QUESTION 25
A physician is treating a 78-year-old woman with metastatic lung cancer. She has decision-making capacity. She tells her physician she does not want aggressive chemotherapy and wishes to focus on comfort care. Her adult son, who holds her healthcare proxy, insists the physician “do everything possible” and demands chemotherapy be initiated. What is the most appropriate course of action?
A) Honor the son’s request as healthcare proxy holder
B) Convene an ethics committee to decide
C) Initiate chemotherapy as a compromise while pursuing further discussion
D) Honor the patient’s expressed wishes, as she retains decision-making capacity
E) Transfer care to a physician willing to provide aggressive treatment
✅ Correct Answer: D — Honor the patient’s expressed wishes, as she retains decision-making capacity
Detailed Explanation:
This is a medical ethics question testing patient autonomy and the appropriate role of healthcare proxies.
The Core Principle: A healthcare proxy (or healthcare power of attorney) is only activated when the patient lacks decision-making capacity. As long as a patient has capacity, they make their own medical decisions — regardless of what family members want.
Decision-Making Capacity requires:
- Understanding the diagnosis, treatment options, and consequences
- Appreciation that the information applies to them
- Reasoning ability to weigh options
- Communication of a consistent choice
This patient has capacity → her wishes supersede the son’s proxy authority. The son has NO authority to override his mother while she can speak for herself.
Healthcare Proxy vs. Power of Attorney:
- Healthcare proxy = medical decisions only, activates on incapacity
- Durable power of attorney = legal/financial decisions
- Living will = written advance directive specifying wishes
What if she lacked capacity? Then the healthcare proxy would be authorized to make decisions — but even then, the proxy must act according to the patient’s known values and previously expressed wishes (substituted judgment standard), not according to their own preferences.
Ethics Framework for USMLE:
- Autonomy — Patient’s right to self-determination (overrides proxy when patient has capacity)
- Beneficence — Act in patient’s best interest
- Non-maleficence — Do no harm
- Justice — Fair allocation of resources
Master Answer Key & Quick Reference
| Q# | Subject | Answer | Core Concept |
|---|---|---|---|
| 1 | Biochemistry | C | Malignant hyperphenylalaninemia (BH4 deficiency) |
| 2 | Genetics | C | Trinucleotide repeat anticipation |
| 3 | Cardiology | A | Long QT — early afterdepolarizations |
| 4 | Genetics/Repro | C | Complete androgen insensitivity syndrome |
| 5 | Cardiology | C | Cardiac tamponade from pericarditis |
| 6 | Cardiology | C | Cor pulmonale from COPD |
| 7 | Pulmonary | B | Hemorrhagic infarction — PE |
| 8 | Cardiology | B | Eisenmenger syndrome |
| 9 | Microbiology | C | Cryptococcal capsule — phagocytosis evasion |
| 10 | Microbiology | C | LPS → TLR-4 → NF-κB → septic shock |
| 11 | Microbiology | C | S. aureus exfoliative toxin — desmoglein-1 |
| 12 | Microbiology | C | Syphilis — benzathine penicillin G |
| 13 | Neuroscience | B | Alzheimer’s — amyloid plaques + NFTs |
| 14 | Neuroscience | C | Temporal lobe focal seizure |
| 15 | Neuroscience | C | ALS — UMN + LMN, no sensory |
| 16 | Endocrine | C | Graves’ — TSH receptor antibodies |
| 17 | Pharmacology | B | SGLT-2 inhibitors — cardiorenal benefits |
| 18 | Hematology | C | G6PD — normal electrophoresis |
| 19 | Oncology | D | CLL — CD5+/CD19+/CD23+ immunophenotype |
| 20 | Oncology | A | Papillary thyroid carcinoma — BRAF/RET |
| 21 | Reproductive | B | Hydatidiform mole → GTN surveillance |
| 22 | GI | C | Esophageal SCC — middle third, alcohol/tobacco |
| 23 | MSK | C | RA pathophysiology — pannus formation |
| 24 | Biostatistics | D | Sensitivity vs. specificity calculation |
| 25 | Ethics | D | Patient autonomy > healthcare proxy |
Your Next Steps: Building On These Sample Questions
These 25 questions are a starting point — but Step 1 demands thousands of exposures across all disciplines. Here’s how to build from here:
Step 1: Use UWorld as your primary question bank. It contains ~3,400 questions with the most detailed, accurate explanations available. Set it to tutor mode initially so you learn from each question immediately.
Step 2: Track your performance by subject using UWorld’s built-in analytics. Any subject below 50% correct deserves a dedicated review session within 48 hours.
Step 3: Convert insights from question explanations into Anki flashcards. The AnKing Overhaul deck is the most comprehensive pre-made Step 1 deck, already mapped to First Aid.
Step 4: Take NBME practice exams on a schedule — your exam scores on these are the most reliable predictor of your actual Step 1 score. UWSA1 and UWSA2 are considered the most predictive within 30 days of the real exam.
Step 5: Come back to this guide periodically. Re-attempt the questions cold. If you can explain every answer choice — not just the correct one — you’re thinking at a level that will serve you well on exam day.
Disclaimer: This content is written for USMLE Step 1 board examination preparation purposes only. It does not constitute clinical medical advice. Always refer to current clinical guidelines and licensed healthcare providers for patient care decisions.