A 48-year-old man presents with fever, heart failure, and a new regurgitation murmur. Blood cultures remain negative after 72 hours. Serology shows phase I IgG antibody titer >1:800. What is the most likely causative organism?
- A Bartonella henselae
- B Tropheryma whipplei
- C Coxiella burnetii ✓
- D HACEK group organisms
Correct answer: C. Coxiella burnetii
Explanation
Coxiella burnetii (Q fever) endocarditis is diagnosed by phase I IgG antibody titer >1:800, which is a major Duke criterion. It is a common cause of culture-negative endocarditis, particularly in patients with prosthetic valves or pre-existing valvular disease. HACEK organisms grow in standard cultures (usually within 5 days), while Bartonella requires different serological criteria.
Reference: Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, 9th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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