This 35-year-old pet shop worker developed progressively spreading nodular lesions. He was afebrile. What is the most likely causative organism?

Answer C. Mycobacterium marinumNeedle aspiration of a nodule recovered thick, purulent fluid that was positive for acid-fast bacilli, identified on culture as M. marinum. The patient was treated successfully with a four-month regimen of rifampin and ethambutol. M. marinum infection is an occupational hazard for aquarium cleaners. Infection is usually limited to the skin, with the hands, elbows, and feet most commonly affected. The appearance is not typical of infection with the other listed organisms.