A 45-year-old IV drug user presents with fever and a new tricuspid regurgitation murmur. Echocardiography shows a large vegetation on the tricuspid valve. Blood cultures grow Staphylococcus aureus. What histological feature MOST distinguishes subacute from acute infective endocarditis?

• A Acute endocarditis lacks fibrin and platelets; subacute endocarditis has pure fibrin thrombi
• B Subacute endocarditis always shows calcification of the valve annulus
• C Acute endocarditis shows sterile vegetations, while subacute has infected vegetations with bacteria
• D Subacute endocarditis has granulation tissue and fibrosis beneath the vegetation indicating repeated infection on pre-existing abnormal valves ✓

Explanation

In subacute infective endocarditis (typically Streptococcus viridans on previously abnormal valves), the prolonged, indolent course allows granulation tissue and fibrosis to develop beneath the vegetation, reflecting organized healing responses. Acute endocarditis (typically S. aureus) destroys previously normal valves rapidly with bulky, destructive vegetations lacking significant underlying repair. Both forms have fibrin-platelet vegetations containing bacteria, but the subacute form shows more organized histological features.

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

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