Preparing for the USMLE Step 1 requires a strong command of microbiology, especially high-yield organisms, toxins, and clinical correlations.

This page features 25 USMLE-style microbiology questions with clear explanations, covering bacteria, viruses, fungi, and parasites frequently tested on exam day. Each question is designed to mirror real USMLE concepts and help reinforce rapid clinical decision-making.

USMLE Microbiology Questions with Explanations (25 High-Yield MCQs)

Question 1

A 22-year-old college student presents with fever, headache, and neck stiffness. CSF shows increased neutrophils and decreased glucose. Gram stain reveals gram-negative diplococci. What is the most important virulence factor?

A) Protein A
B) Capsule
C) Exotoxin A
D) Lipoteichoic acid

Answer: B) Capsule

Explanation:
This is Neisseria meningitidis. The polysaccharide capsule prevents phagocytosis and is the key virulence factor.

Question 2

A patient develops bloody diarrhea after eating undercooked ground beef. Stool culture shows sorbitol-negative gram-negative rods. Mechanism of disease?

A) Heat-stable toxin
B) Shiga-like toxin inhibiting 60S ribosome
C) Invasion of Peyer patches
D) cAMP activation

Answer: B) Shiga-like toxin inhibiting 60S ribosome

Explanation:
This is EHEC (O157:H7). It produces Shiga-like toxin that inhibits the 60S ribosomal subunit → HUS risk.

Question 3

A child presents with barking cough and inspiratory stridor. Cause?

A) RSV
B) Parainfluenza virus
C) Influenza
D) Adenovirus

Answer: B) Parainfluenza virus

Explanation:
Parainfluenza causes croup (laryngotracheobronchitis).

Question 4

A patient with HIV has pneumonia. Silver stain shows cup-shaped cysts. Organism?

A) Histoplasma
B) Cryptococcus
C) Pneumocystis jirovecii
D) Aspergillus

Answer: C) Pneumocystis jirovecii

Question 5

Which organism produces alpha toxin that destroys cell membranes?

A) Bacillus anthracis
B) Clostridium perfringens
C) Clostridium tetani
D) Listeria monocytogenes

Answer: B) Clostridium perfringens

Explanation:
Alpha toxin is a lecithinase causing gas gangrene.

Question 6

A patient develops ascending paralysis after eating improperly canned food. Mechanism?

A) Blocks acetylcholine release
B) Blocks glycine release
C) Stimulates acetylcholine release
D) Destroys Schwann cells

Answer: A) Blocks acetylcholine release

Explanation:
Botulinum toxin prevents ACh release → flaccid paralysis.

Question 7

Which virus integrates into host DNA using reverse transcriptase?

A) Influenza
B) HIV
C) HSV
D) CMV

Answer: B) HIV

Question 8

A painless genital ulcer with indurated edges. Diagnosis?

A) HSV
B) Syphilis
C) Chancroid
D) Gonorrhea

Answer: B) Syphilis

Explanation:
Treponema pallidum causes painless chancre.

Question 9

Most common cause of atypical pneumonia in young adults?

A) Streptococcus pneumoniae
B) Mycoplasma pneumoniae
C) Klebsiella
D) Legionella

Answer: B) Mycoplasma pneumoniae

Question 10

Which organism grows on Thayer-Martin agar?

A) N. meningitidis
B) N. gonorrhoeae
C) E. coli
D) H. influenzae

Answer: B) N. gonorrhoeae

Question 11

A patient develops pseudomembranous colitis after antibiotics. Toxin mechanism?

A) Inhibits EF-2
B) Activates adenylate cyclase
C) Inactivates Rho GTPases
D) Blocks 60S ribosome

Answer: C) Inactivates Rho GTPases

Explanation:
C. difficile toxin damages cytoskeleton → colitis.

Question 12

Which organism has a polysaccharide capsule detected by India ink?

A) Candida
B) Histoplasma
C) Cryptococcus neoformans
D) Aspergillus

Answer: C) Cryptococcus

Question 13

Which bacterium is urease positive and associated with peptic ulcers?

A) Campylobacter
B) H. pylori
C) Vibrio cholerae
D) Salmonella

Answer: B) H. pylori

Question 14

A patient with splenectomy is most at risk for infection with:

A) Gram-positive rods
B) Encapsulated organisms
C) Anaerobes
D) Mycobacteria

Answer: B) Encapsulated organisms

Question 15

Which virus causes Koplik spots?

A) Rubella
B) Measles
C) Varicella
D) EBV

Answer: B) Measles

Question 16

A rusty sputum pneumonia suggests:

A) Klebsiella
B) Staph aureus
C) Strep pneumoniae
D) Pseudomonas

Answer: C) Strep pneumoniae

Question 17

Which organism causes rice-water diarrhea?

A) ETEC
B) Vibrio cholerae
C) Shigella
D) Salmonella

Answer: B) Vibrio cholerae

Question 18

Which bacteria is acid-fast?

A) Listeria
B) Mycobacterium tuberculosis
C) Nocardia (partial)
D) Both B and C

Answer: D) Both B and C

Question 19

A patient has bull’s-eye rash after tick bite. Organism?

A) Rickettsia
B) Borrelia burgdorferi
C) Babesia
D) Ehrlichia

Answer: B) Borrelia burgdorferi

Question 20

Which fungus forms hyphae branching at 45 degrees?

A) Mucor
B) Candida
C) Aspergillus
D) Histoplasma

Answer: C) Aspergillus

Question 21

A newborn develops conjunctivitis 5 days after birth. Cause?

A) HSV
B) Gonorrhea
C) Chlamydia
D) CMV

Answer: C) Chlamydia

Question 22

Which parasite causes macrocytic anemia?

A) Giardia
B) Entamoeba
C) Diphyllobothrium latum
D) Toxoplasma

Answer: C) Diphyllobothrium latum

Question 23

Which virus causes fifth disease?

A) Parvovirus B19
B) CMV
C) HHV-6
D) EBV

Answer: A) Parvovirus B19

Question 24

Which bacteria produces superantigen toxin causing toxic shock syndrome?

A) Streptococcus pyogenes
B) Staph aureus
C) Both A and B
D) Clostridium

Answer: C) Both A and B

Question 25

Which organism is oxidase positive and produces blue-green pigment?

A) E. coli
B) Pseudomonas aeruginosa
C) Klebsiella
D) Proteus

Answer: B) Pseudomonas aeruginosa

❓ Frequently Asked Questions

Q1. Are microbiology questions high yield for USMLE Step 1?
Yes. Microbiology is heavily tested, especially toxins, virulence factors, and classic clinical scenarios.

Q2. Is this content suitable for international medical graduates (IMGs)?
Yes. These questions follow standard USMLE exam patterns used for both US and international candidates.

Q3. How many microbiology questions appear on USMLE Step 1?
Microbiology accounts for a significant portion of infectious disease and pathology questions across multiple systems.

USMLE Microbiology Questions with Explanations – 25 High-Yield MCQs