NBME Microbiology 2 ā Questions and Answers
Question 1: A patient develops watery diarrhea after eating reheated fried rice at a picnic. The causative organism produces a heat-stable toxin that activates adenylyl cyclase. Which organism is most likely responsible?
- Bacillus cereus (Correct answer)
- Staphylococcus aureus
- Clostridium perfringens
- Vibrio cholerae
Correct answer: Bacillus cereus
Bacillus cereus emetic toxin is heat-stable and associated with reheated rice; its diarrheal toxin activates adenylyl cyclase similar to cholera toxin.
Question 2: A Gram-negative diplococcus is isolated from urethral discharge of a sexually active male. The organism grows on Thayer-Martin agar and is oxidase-positive. Which virulence factor allows this organism to evade phagocyte killing?
- IgA protease (Correct answer)
- Protein A
- Leukocidin
- Coagulase
Correct answer: IgA protease
Neisseria gonorrhoeae produces IgA protease, which cleaves secretory IgA and helps the organism evade mucosal immune defenses.
Question 3: A 55-year-old rancher develops a painless black eschar on his forearm after handling livestock hides. Blood cultures grow large, box-car-shaped Gram-positive rods. Which structure is primarily responsible for antiphagocytic activity of this pathogen?
- Poly-D-glutamate capsule (Correct answer)
- Protein A
- M protein
- Vi antigen
Correct answer: Poly-D-glutamate capsule
Bacillus anthracis has a poly-D-glutamate capsule that inhibits phagocytosis; this unusual D-amino acid capsule is not recognized by host proteases.
Question 4: A laboratory test reveals that an organism's cell wall lacks peptidoglycan and instead contains mycolic acids. This organism would be expected to stain positive with which technique?
- Acid-fast stain (Correct answer)
- Gram stain
- India ink
- Silver stain
Correct answer: Acid-fast stain
Mycobacteria contain mycolic acids that bind carbol-fuchsin and resist decolorization with acid-alcohol, making them acid-fast positive.
Question 5: A child presents with honey-crusted lesions around the nose and mouth. Culture grows Gram-positive cocci in clusters. The organism produces a toxin that cleaves desmoglein-1. Which toxin is responsible?
- Exfoliative toxin (Correct answer)
- TSST-1
- Alpha toxin
- Leukocidin
Correct answer: Exfoliative toxin
Staphylococcus aureus exfoliative toxin (epidermolytic toxin) cleaves desmoglein-1 in the stratum granulosum, causing skin separation in impetigo and scalded skin syndrome.
Question 6: A 30-year-old woman develops a high fever, diffuse erythematous rash, and hypotension during menses. Blood cultures are negative. Which mechanism best explains systemic toxicity in this condition?
- Superantigen cross-linking MHC II and T-cell receptor (Correct answer)
- Lipopolysaccharide binding TLR-4
- Type III hypersensitivity immune complex deposition
- Exotoxin blocking acetylcholine release
Correct answer: Superantigen cross-linking MHC II and T-cell receptor
TSST-1 is a superantigen that cross-links MHC class II on APCs with the VĪ² region of T-cell receptors, causing massive non-specific T-cell activation and cytokine storm.
Question 7: Which organism is responsible for gas gangrene and produces an alpha toxin (lecithinase) that destroys cell membranes by cleaving phospholipids?
- Clostridium perfringens (Correct answer)
- Clostridium tetani
- Clostridium botulinum
- Clostridium difficile
Correct answer: Clostridium perfringens
Clostridium perfringens produces alpha toxin (a phospholipase C/lecithinase) that hydrolyzes phosphatidylcholine in cell membranes, causing myonecrosis and hemolysis.
NBME Microbiology 2
A patient develops watery diarrhea after eating reheated fried rice at a picnic.
The causative organism produces a heat-stable toxin that activates adenylyl cyclase.
Which organism is most likely responsible?