A 26-year-old aquarium hobbyist develops a slowly growing violaceous plaque on the dorsum of his right hand over several weeks, with sporotrichoid spread along the forearm. Biopsy shows granulomatous inflammation with sparse acid-fast bacilli. Culture grows colonies after 2 weeks at 31 degrees Celsius but not at 37 degrees Celsius. The causative organism is:

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

Explanation

Mycobacterium marinum, a Runyon group I photochromogen, prefers temperatures of 30 to 33 degrees Celsius and fails to grow reliably at 37 degrees, which explains its confinement to cooler superficial tissues of the skin. It causes fish-tank or swimming-pool granuloma with sporotrichoid nodular spread. M. fortuitum grows rapidly within a week, while M. ulcerans produces painless deep ulcers rather than plaques.

Reference: Fitzpatrick's Dermatology, 9th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.