A 22-year-old college student develops meningitis. CSF Gram stain shows lancet-shaped Gram-positive diplococci. Which virulence factor is most critical for this organism's pathogenicity?

• A M protein preventing complement deposition
• B Pneumolysin forming pores in host cell membranes
• C Polysaccharide capsule resisting phagocytosis ✓
• D IgA protease facilitating mucosal invasion

Explanation

Streptococcus pneumoniae's polysaccharide capsule is the single most important virulence factor, rendering it resistant to phagocytosis. Capsular serotypes determine pathogenicity and form the basis of the pneumococcal vaccine. M protein is a Group A Streptococcus virulence factor. Pneumolysin and IgIA protease contribute to pathogenesis but are secondary to the capsule. The lancet-shaped diplococci morphology is characteristic of S. pneumoniae in CSF.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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