Which of the following is a major criterion in the Duke criteria for diagnosing infective endocarditis?

• A. Positive blood cultures
• B. Fever
• C. Predisposing condition
• D. Vascular phenomenon

Answer: (A)

The key idea is that definite microbiologic evidence from the bloodstream is the strongest sign of infective endocarditis. In the Duke criteria, positive blood cultures for organisms typical of endocarditis—and, importantly, from two separate cultures or persistently positive cultures—constitute a major criterion. This shows active infection of the endocardium on a microbiologic level, which is why it’s considered major. Other features like fever, a predisposing heart condition, or vascular phenomena are included in the framework as minor criteria; they raise suspicion but don’t prove endocarditis on their own. Diagnosing IE relies on combining criteria, such as two major criteria or one major plus three minor, etc. So the presence of positive blood cultures is the best answer because it directly meets the major criterion.