A 65-year-old patient in the ICU develops ventilator-associated pneumonia. Sputum culture grows Gram-positive cocci in chains that are catalase-negative, optochin-sensitive, and bile-soluble. A Quellung reaction is positive. Which virulence factor is most critical for the pathogenicity of this organism?

• A Pneumolysin
• B Pili
• C IgA protease
• D Polysaccharide capsule ✓

Explanation

The organism is Streptococcus pneumoniae, identified by optochin sensitivity, bile solubility, and a positive Quellung reaction (capsular swelling with specific antiserum). The polysaccharide capsule is the single most important virulence factor of S. pneumoniae, as it inhibits complement-mediated opsonophagocytosis. Pneumolysin and IgA protease contribute to virulence, but the capsule is the primary determinant of pathogenicity and the target of pneumococcal vaccines.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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