Bacteriology USMLE Step 1 Practice Question
A 45-year-old man with a 20 pack-year smoking history and COPD presents to the emergency department with a 3-day history of productive cough with purulent sputum, dyspnea, and fever. Vital signs show temperature 38.8°C, respiratory rate 24/min, and oxygen saturation 88% on room air. Chest X-ray demonstrates a left lower lobe consolidation. Gram stain of expectorated sputum reveals gram-negative diplococci. Blood and sputum cultures are pending. Which of the following virulence factors is most responsible for this organism's ability to avoid complement-mediated killing and neutrophil opsonization?
Answer choices
- AFlagellar appendages that promote mucociliary escape and distal airway seeding
- BType IV pili mediating DNA uptake and spontaneous antibiotic resistance acquisition
- CEndotoxin (lipopolysaccharide) triggering excessive inflammatory cytokine release
- DPolysaccharide capsule masking pathogen-associated molecular patterns from immune recognitionCorrect answer
- EIgA1 protease enzymatic cleavage of secretory immunoglobulin in respiratory secretions
- FHeat-labile exotoxin directly lysing ciliated epithelial cells and impairing clearance
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