A 24-year-old pet shop employee develops a single violaceous nodule on the dorsum of his right hand, followed by an ascending chain of similar nodules along the forearm over 4 weeks. He has no fever. Biopsy shows granulomatous inflammation with sparse acid-fast bacilli. Culture grows mycobacteria optimally at 31°C. What is the causative organism?

• A Mycobacterium haemophilum
• B Mycobacterium kansasii
• C Mycobacterium marinum ✓
• D Mycobacterium ulcerans

Explanation

Fish tank granuloma is caused by M. marinum, a photochromogen found in fresh and salt water. It grows best at 30 to 33°A, explaining its predilection for cooler skin sites such as the extremities, and it fails to grow at 37°A on routine cultures. Sporotrichoid ascending nodules mimic Sporothrix infection. The history of aquatic exposure plus low-temperature growth separates it from M. haemophilum, which also prefers cooler temperatures but occurs mainly in immunocompromised hosts.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.