Microbiology Flashcards
7 cards from real NBME practice questions. Tap to flip, then mark Knew It or Still Learning ā missed cards come back until you master them.
Read the first 7 Microbiology flashcards as text
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?
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.
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?
Answer: IgA protease
Neisseria gonorrhoeae produces IgA protease, which cleaves secretory IgA and helps the organism evade mucosal immune defenses.
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?
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.
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?
Answer: Acid-fast stain
Mycobacteria contain mycolic acids that bind carbol-fuchsin and resist decolorization with acid-alcohol, making them acid-fast positive.
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?
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.
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?
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.
Which organism is responsible for gas gangrene and produces an alpha toxin (lecithinase) that destroys cell membranes by cleaving phospholipids?
Answer: Clostridium perfringens
Clostridium perfringens produces alpha toxin (a phospholipase C/lecithinase) that hydrolyzes phosphatidylcholine in cell membranes, causing myonecrosis and hemolysis.