American Board of Internal Medicine (ABIM) Certification Practice Exam

What is the common presentation of Mycobacterium marinum infection?

1. Fever with rashes

2. Ulcerative lesion with central eschar

3. Ascending lymphatic spread with systemic toxicity

4. Multiple boils and furuncles

The correct answer is: Ulcerative lesion with central eschar

Mycobacterium marinum infection is typically associated with skin lesions that manifest as ulcerative lesions, particularly those with a central eschar. This organism is known to cause “swimming pool granuloma” or “fish tank granuloma,” which often results from exposure to contaminated water environments, such as aquariums or swimming pools. The classic presentation usually involves a small, raised, red bump that progressively enlarges, often developing into an ulcer with a characteristic central eschar. This lesion commonly appears on the extremities and may take several weeks to manifest after exposure. The chronic nature of the infection and the ulcerative aspect are distinctive features that set it apart from other cutaneous infections caused by different organisms. Understanding this disease process is crucial as it helps differentiate it from other types of skin infections with different presentations. For example, while systemic involvement or other forms of skin infection may be a consequence of other pathogens, Mycobacterium marinum is particularly noted for localized ulcerative skin lesions.