Pathological examination of a cardiac valve removed during surgery for infective endocarditis shows large, irregular, destructive vegetations with underlying valve leaflet necrosis. Gram stain of the vegetation shows Gram-positive cocci in clusters. The underlying valvular predisposing lesion most commonly associated with S. aureus endocarditis is:
- A Previously normal valves — S. aureus infects structurally normal valves ✓
- B Rheumatic heart disease with mitral stenosis
- C Mitral valve prolapse
- D Bicuspid aortic valve
Explanation
Staphylococcus aureus is unique among IE pathogens in its ability to infect structurally normal, previously undamaged cardiac valves — typically the aortic or mitral valve. It produces virulence factors including protein A (inhibiting opsonization), TSST-1, coagulase, and fibronectin-binding proteins that facilitate direct valve colonization without prior endothelial injury. Viridans streptococci typically require prior valve damage (NBTE, rheumatic disease). This explains why S. aureus IE is more common in IV drug users affecting tricuspid valves, or from catheter-related bacteremia on normal left-sided valves.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.