Which organism causes granulomatous skin lesions after aquarium or fish tank exposure?

• A. Mycobacterium marinum
• B. Mycobacterium leprae
• C. Nocardia asteroides
• D. Pseudomonas aeruginosa

Answer: (A)

Aquarium exposure is a classic scenario for a skin infection caused by Mycobacterium marinum, a non-tuberculous mycobacterium that lives in aquatic environments. When the skin is breached during fish tank cleaning or handling, this organism can enter and establish a chronic granulomatous lesion on the hands or forearms. The lesions typically begin as papules or nodules and can ulcerate, sometimes spreading along lymphatic channels in a sporotrichoid pattern. Diagnosis is supported by culture at cooler temperatures (around 30°C) and identification of acid-fast bacilli, with the organism being slow-growing. Treatment usually requires multi-drug therapy for several months, commonly rifampin combined with ethambutol, and may include clarithromycin depending on the case. Other organisms listed are associated with different diseases or exposure settings (for example, leprosy with Mycobacterium leprae, nocardial infections from soil, or Pseudomonas-related hot tub folliculitis), so the link to aquarium-associated granulomatous skin lesions best fits Mycobacterium marinum.