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This Clinical Problem-Solving article presents a diagnostically complex case involving a 75-year-old man with a pruritic leg rash, malaise, and renal dysfunction. Initially suspected to have hepatitis C–associated mixed cryoglobulinemic vasculitis, the patient was later diagnosed with Bartonella henselae endocarditis, based on high IgG titers and vegetations on transesophageal echocardiography. He also had hypocomplementemic glomerulonephritis and leukocytoclastic vasculitis on renal and skin biopsies. The case illustrates how heuristics and confirmation bias can mislead clinical reasoning and emphasizes the importance of diagnostic thoroughness, especially in culture-negative endocarditis. Treatment involved doxycycline and rifampin, with discontinuation of hepatitis C antivirals to avoid drug interaction, resulting in resolution of symptoms and improved renal function.

This Clinical Problem-Solving article presents a diagnostically complex case involving a 75-year-old man with a pruritic leg rash, malaise, and renal dysfunction. Initially suspected to have hepatitis C–associated mixed cryoglobulinemic vasculitis, the patient was later diagnosed with Bartonella henselae endocarditis, based on high IgG titers and vegetations on transesophageal echocardiography. He also had hypocomplementemic glomerulonephritis and leukocytoclastic vasculitis on renal and skin biopsies. The case illustrates how heuristics and confirmation bias can mislead clinical reasoning and emphasizes the importance of diagnostic thoroughness, especially in culture-negative endocarditis. Treatment involved doxycycline and rifampin, with discontinuation of hepatitis C antivirals to avoid drug interaction, resulting in resolution of symptoms and improved renal function.