A 45-year-old patient with HIV (CD4 = 75 cells/µL) develops fever, weight loss, diarrhoea, and elevated ALP. Blood culture yields acid-fast bacilli growing at 25–37°C on Löwenstein-Jensen medium. The organism forms smooth, nonpigmented colonies and does NOT produce niacin. The most likely organism and treatment are:

• A M. tuberculosis — standard RHEZ anti-TB regimen
• B M. kansasii — isoniazid + rifampicin + ethambutol for 12–18 months
• C Mycobacterium avium complex (MAC) — clarithromycin + ethambutol ± rifabutin ✓
• D M. fortuitum — amikacin + imipenem + ciprofloxacin

Explanation

Mycobacterium avium complex (MAC), including M. avium and M. intracellulare, is the most common opportunistic mycobacterial infection in advanced HIV disease (CD4 <50 cells/µL). MAC causes disseminated disease — bacteraemia, hepatosplenomegaly, elevated ALP, anaemia, and wasting. It is niacin-negative (unlike M. tuberculosis which is niacin-positive), grows at 37°C, and is generally resistant to standard TB drugs. Treatment is clarithromycin (or azithromycin) + ethambutol ± rifabutin for at least 12 months (and until CD4 >100 for ≥6 months on ART). Prophylaxis with azithromycin is indicated when CD4 <50 cells/µL. M. kansasii produces a yellow pigment in light (photochromogen).

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.