Which major criterion indicates endocardial involvement in infective endocarditis?

Endocardial involvement in infective endocarditis is primarily indicated by findings on an echocardiogram, which can show vegetations, abscesses, or other structural changes associated with the infection of the heart valves or endocardium. An echocardiogram is a critical diagnostic tool that directly visualizes the heart structures and provides evidence of the disease's presence on the endocardium. This imaging can help to confirm the diagnosis and assess the extent of the infection, making it a crucial criterion in diagnosing infective endocarditis.

While positive blood cultures, fever, and vascular phenomena are important features in the diagnosis of infective endocarditis, they do not specifically indicate endocardial involvement. Positive blood cultures demonstrate the presence of bacteria but do not show where the infection is located. Fever is a common symptom in many infections and does not specifically indicate heart involvement. Vascular phenomena may suggest complications of the disease but do not visualize the endocardial structure itself. Therefore, the echocardiographic findings are the definitive criteria to confirm endocardial involvement in infective endocarditis.