A 42-year-old HIV-positive patient (CD4 120 cells/µL) presents with fever, weight loss, and diffuse reticulonodular infiltrates on chest X-ray. Bone marrow aspirate culture grows acid-fast bacilli. The isolate is niacin-negative, nitrate reduction-negative, and TCH (thiophene-2-carboxylic acid hydrazide) positive. Which organism is most likely?
- A Mycobacterium bovis ✓
- B Mycobacterium tuberculosis
- C Mycobacterium avium complex
- D Mycobacterium kansasii
Correct answer: A. Mycobacterium bovis
Explanation
M. bovis is niacin-negative, nitrate-reduction-negative, and TCH-positive, distinguishing it from M. tuberculosis (niacin-positive, nitrate-positive, TCH-negative). In HIV patients with low CD4, disseminated MAC is common, but MAC is TCH-negative and grows poorly at 37°C without specific biochemicals. M. kansasii is photochromogenic, niacin-negative, but nitrate-positive. The TCH positivity is the key discriminator for M. bovis.
Reference: Mackie and McCartney Practical Medical Microbiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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