A photochromogenic mycobacterium is isolated from the sputum of a patient with chronic cavitary lung disease. The isolate is niacin-negative and nitrate-negative. Which organism is most likely?

• A Mycobacterium tuberculosis
• B Mycobacterium fortuitum
• C Mycobacterium avium complex
• D Mycobacterium kansasii ✓

Explanation

Mycobacterium kansasii is a slow-growing photochromogen causing TB-like cavitary disease. It is niacin and nitrate negative, unlike M. tuberculosis which is positive for both. MAC is non-photochromogenic. M. fortuitum is a rapid grower. The biochemical profile matches M. kansasii.

Reference: Koneman's Color Atlas and Textbook of Diagnostic Microbiology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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