Which organism is the etiologic agent of swimming pool granuloma?

• A. M. fortuitum
• B. M. kansasii
• C. M. marinum
• D. M. xenopi

Answer: (C)

Swimming pool granuloma is caused by infection of the skin with Mycobacterium marinum, a non-tuberculous mycobacterium that lives in aquatic environments such as swimming pools and fish tanks. It often follows minor skin trauma and presents as nodules on the hands or forearms that may ulcerate. In the lab, M. marinum grows best at cooler temperatures (around 30°C) and is photochromogenic, producing a yellow pigment when exposed to light—traits that help distinguish it from other mycobacteria. The other organisms listed are different non-tuberculous mycobacteria with other typical disease patterns (often pulmonary or non-pool-associated skin infections) and are not the classic cause of swimming pool granuloma.