A 30-year-old man develops a pustular lesion at the site of a cat scratch on his forearm. This is followed 1 week later by malaise, fever, and lymphadenopathy. On examination, he has tender axillary lymph nodes. Which of the following is the most likely causative organism?

The cause of cat-scratch fever is a tiny Gram-negative bacillus, B. henselae. Cats acquire the organism from the soil and inoculate humans via scratches or bites. The disease is generally benign and self-limited, and is treated with analgesics and antipyretics. Encephalitis, seizures, coma, meningitis, and transverse myelitis can occasionally occur even in immunocompetent patients. A variety of antibiotics have been used when severe disease is present but an optimal regimen has not been identified.