A nurse is reviewing laboratory results for a client with suspected endocarditis. Which finding would confirm the diagnosis?

Infective endocarditis is a microbial infection of the endothelial surface of the heart, most commonly affecting the heart valves. Diagnosis relies heavily on the Duke Criteria, which combine clinical, echocardiographic, and microbiological findings to provide a definitive diagnosis of the presence of vegetations and systemic infection. A. Low-density lipoproteins are markers used to assess cardiovascular risk and atherosclerosis. While high levels contribute to coronary artery disease, they have no diagnostic value in identifying an infectious process within the heart. They represent a metabolic and nutritional status rather than an acute or chronic inflammatory or infectious condition. B. Positive blood cultures are considered a major Duke Criterion and are essential for confirming endocarditis. Identifying the specific pathogen in the bloodstream not only confirms the infectious nature of the disease but also guides the selection of targeted bactericidal antibiotic therapy required to penetrate the avascular vegetations on the heart valves. C. Elevated cardiac troponin levels are highly specific markers for myocardial injury, typically seen in acute coronary syndromes like myocardial infarction. While severe endocarditis can occasionally cause secondary myocardial irritation or abscess formation, troponins are not a primary diagnostic tool for confirming endocardial infection or valvular vegetations. D. An increased white blood cell count, or leukocytosis, is a non-specific indicator of inflammation or infection anywhere in the body. While it is commonly found in patients with endocarditis, it does not confirm the diagnosis because it could be caused by various other conditions, ranging from simple UTIs to systemic trauma.