American Board of Internal Medicine (ABIM) Certification Practice Exam

Which organism is primarily associated with lesions resulting from exposure to fresh water or salt water?

1. Francisella tularensis

2. Mycobacterium fortuitum

3. Mycobacterium marinum

4. Zika virus

The correct answer is: Mycobacterium marinum

The organism that is primarily associated with lesions resulting from exposure to fresh water or salt water is Mycobacterium marinum. This species is known to cause skin infections, particularly in individuals who have had contact with contaminated water sources, such as aquariums, fish tanks, or when swimming in fresh or salt water. Mycobacterium marinum is a non-tuberculous mycobacterium that can enter through breaks in the skin, leading to a characteristic infection often referred to as "swimming pool granuloma" or "fish tank granuloma." It typically presents as nodular lesions that can progress to ulcers or abscesses. Knowledge of this organism is vital for clinicians, especially when evaluating patients with such cutaneous lesions in the correct context of water exposure. In contrast, Francisella tularensis is more associated with rabits and ticks, leading to tularemia; Mycobacterium fortuitum is related to infections from contact with contaminated medical equipment or environmental sources; and Zika virus is primarily transmitted by mosquitoes and associated with viral infections. Understanding these differences helps clarify why Mycobacterium marinum stands out as the correct answer in the context of water-related lesions.