Your friendly neighborhood spider bite - Or not: A fishy case of Mycobacterium marinum.

Mycobacterium marinum is a slow-growing nontuberculous mycobacterium, most commonly causing cutaneous infection after prolonged or repeated exposure to contaminated aquatic environments. Diagnosis is frequently delayed because of its indolent course, nonspecific clinical features, and frequent misattribution to more common dermatologic conditions. We report the case of a 51-year-old immunocompromised woman who developed a progressive ulcerative lesion on the right wrist after a presumed spider bite following a farm visit. Despite multiple emergency department encounters and empiric antibacterial therapy, the lesion failed to improve. Acid-fast bacilli culture from a wound specimen ultimately grew M. marinum several weeks after initial presentation. Subsequent history revealed a previously undisclosed history of exposure to a home aquarium. Management was complicated by impaired oral antimicrobial absorption following prior bariatric surgery and incomplete clinical response to initial therapy, necessitating prolonged outpatient parenteral antimicrobial treatment, with eventual clinical resolution. This case underscores the impact of diagnostic anchoring bias, highlights the importance of thorough environmental exposure assessment beyond classic aquatic settings, and emphasizes early consideration of nontuberculous mycobacterial infection in chronic, nonhealing cutaneous lesions.