A 45-year-old patient with HIV (CD4 = 60) develops fever, night sweats, and markedly elevated alkaline phosphatase. Blood culture in BACTEC MYCO/F Lytic bottle grows slowly at 37°C and on Middlebrook agar shows flat, rough, non-pigmented colonies. ZN stain shows short acid-fast bacilli, negative for pigment. What is the most likely organism?
- A Mycobacterium tuberculosis
- B Mycobacterium kansasii
- C Mycobacterium avium complex (MAC) ✓
- D Mycobacterium marinum
Correct answer: C. Mycobacterium avium complex (MAC)
Explanation
MAC (M. avium, M. intracellulare) is the most common disseminated opportunistic mycobacterial infection in HIV patients with CD4 <50; blood cultures using lytic BACTEC bottles are the most sensitive diagnostic method. Colonies are flat and non-pigmented in contrast to M. kansasii (photochromogen, yellow pigment on exposure to light) and M. marinum (photochromogen, associated with fish tanks/swimming pools). M. tuberculosis typically grows more slowly and differs epidemiologically.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.