Three days after receiving an intramuscular injection at a local clinic, a 40-year-old man develops a painful abscess at the injection site with purulent discharge. Ziehl-Neelsen stain of pus shows acid-fast bacilli, and cultures show visible colonies within 5 days on Lowenstein-Jensen medium. Which Runyon group does this organism belong to?

• A Group I, photochromogens
• B Group II, scotochromogens
• C Group III, nonchromogens
• D Group IV, rapid growers ✓

Explanation

Visible growth within 7 days defines the Runyon group IV rapid growers, exemplified by Mycobacterium fortuitum and M. chelonae. These saprophytes found in water and soil classically cause post-injection abscesses and post-surgical wound infections following contaminated procedures. Groups I, II and III are slow-growing organisms that require weeks to form colonies on solid media, so the short time to visibility alone separates this isolate from all other Runyon groups.

Reference: Ananthanarayan and Paniker Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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