Blood cultures are negative after repeated sampling in a 55-year-old man with progressive aortic regurgitation, low-grade fever, and vegetations on echocardiography. Serology shows high titers of phase I IgG antibodies against an intracellular gram-negative coccobacillus. The causative agent is:

• A Brucella abortus
• B Coxiella burnetii ✓
• C Legionella pneumophila
• D Tropheryma whipplei

Explanation

Coxiella burnetii, the agent of Q fever, is one of the leading causes of culture-negative infective endocarditis worldwide. Because it is an obligate intracellular organism, standard blood cultures remain negative, and diagnosis rests on serology showing high phase I IgG titers. Brucella and Legionella can occasionally cause culture-negative endocarditis but are rarer, and Tropheryma whipplei is an uncommon cause detected by PCR. Culture-negative workup always includes Coxiella and Bartonella serology.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.