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Mycobacterium Marinum — SCE Dermatology MCQ

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ModerateCutaneous InfectionMycobacterium MarinumSCE Dermatology

A 60-year-old man presents with a painful subcutaneous nodule on his lower leg. He works in a tropical fish aquarium shop. The nodule has been present for 6 weeks, is violaceous, and has developed a small draining sinus. He has a linear chain of similar nodules tracking up his arm. Biopsy shows granulomatous inflammation. What is the most likely causative organism?

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Correct answer: A — Mycobacterium marinum

Mycobacterium marinum ('fish tank granuloma') is an atypical mycobacterium acquired through skin inoculation from contaminated water (fish tanks, swimming pools, marine injuries). It presents as a granulomatous nodule at the inoculation site that may develop sporotrichoid spread (linear chain of nodules along lymphatics). Incubation is 2-8 weeks. It grows optimally at 30-32°C (explaining predilection for cooler extremities). Biopsy shows granulomatous inflammation; culture requires specific conditions (lower temperature, prolonged incubation). Treatment is Clarithromycin + Ethambutol or Rifampicin for 3-6 months. Sporotrichosis (S. schenckii) has an identical morphology but follows plant/soil inoculation.

Reference: BAD 2016; NICE CKS Atypical Mycobacteria