In a patient with a history of IV drug use and suspected infectious endocarditis presenting with fever and Janeway lesions, which finding is most likely to confirm the diagnosis?

• A. Gram-positive cocci seen in the patient's blood culture
• B. Hemorrhagic exudates seen on the patient's retina
• C. Hyperpigmented linear lesions under the patient's nails
• D. Subcutaneous nodules seen over the patient's distal phalanx

Answer: (A)

Infective endocarditis is confirmed most reliably by persistent bacteremia with a microorganism known to cause the disease, demonstrated by positive blood cultures. In an IV drug user, Staphylococcus aureus is the most common culprit, so finding Gram-positive cocci in the blood strongly supports the diagnosis. The other signs listed—retinal Roth spots, splinter hemorrhages under the nails, and Osler nodes—are supportive clues of endocarditis but are not definitive on their own. Positive blood cultures for a typical organism, often alongside echocardiographic evidence of vegetations, meet the major criteria used to confirm the diagnosis.