A 35-year-old IV drug user presents with fever, new murmur, and positive blood cultures for Staphylococcus aureus. Echocardiogram shows 1.5 cm vegetation on the tricuspid valve. Which Duke criterion does a positive blood culture for S. aureus from 2 separate samples satisfy?

• A Minor criterion — fever > 38°C
• B Minor criterion — typical organism from community acquisition
• C Major criterion — only if vegetation confirmed on echo
• D Major criterion — typical microorganism from 2 blood cultures ✓

Explanation

The Modified Duke Criteria for infective endocarditis includes S. aureus bacteremia as a major microbiological criterion (a 'typical' microorganism from 2 separate blood cultures). Other typical organisms (viridans streptococci, S. bovis, HACEK group, Enterococcus in absence of primary focus) also qualify. The echo finding (vegetation, paravalvular abscess, or new prosthetic dehiscence) constitutes a separate major criterion under 'evidence of endocardial involvement.' This patient meets 2 major criteria (positive blood cultures + echocardiographic evidence), satisfying 'definite IE' by Duke criteria.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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