A 35-year-old HIV-positive patient (CD4 count 40 cells/µL) presents with fever, weight loss, and hepatosplenomegaly. Blood culture on BACTEC 13A medium grows acid-fast bacilli after 3 weeks. The isolate grows at 37°C and 42°C but not 25°C, and produces smooth, non-pigmented colonies. It is resistant to isoniazid and susceptible to clarithromycin. Which organism is this?

• A Mycobacterium kansasii
• B Mycobacterium marinum
• C Mycobacterium fortuitum
• D Mycobacterium avium complex (MAC) ✓

Explanation

Mycobacterium avium complex (MAC) is the most common disseminated non-tuberculous mycobacterial (NTM) infection in HIV patients with CD4 count below 50 cells/µL. MAC grows slowly (2–4 weeks), is non-pigmented, and is intrinsically resistant to most first-line anti-TB drugs including isoniazid. Treatment is clarithromycin + ethambutol ± rifabutin. M. kansasii causes photochromogenic (yellow pigment on light exposure) pulmonary disease. M. marinum grows at 25–30°C (fish tank granuloma). M. fortuitum is a rapid grower (rapid-grower NTM).

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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