Bacteriology USMLE Step 1 Practice Question
A 45-year-old man with COPD presents to the emergency department with a 3-day history of productive cough with green sputum, dyspnea, and fever. Vital signs: BP 128/82 mmHg, HR 102/min, RR 24/min, temperature 38.9°C, SpO2 92% on room air. Chest X-ray shows left lower lobe consolidation. Sputum culture yields a gram-negative rod that is oxidase-positive with blue-green pigmentation on blood agar and optimal growth at 42°C. Which of the following virulence factors is most important for this organism's ability to evade host defenses and cause tissue destruction in respiratory infections?
Answer choices
- AExotoxin A (inactivates elongation factor-2) and elastase (degrades elastic fibers)Correct answer
- BLipopolysaccharide endotoxin and type IV pili for mucosal adherence
- CType III secretion system delivering cytotoxins directly into host epithelial cells
- DProtein A binding immunoglobulin Fc regions and M protein molecular mimicry
- EStreptolysins and hyaluronidase for tissue invasion and spread
- FCapsular polysaccharide and peptidoglycan cross-linking only
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