A 32-year-old HIV-positive patient (CD4 count 45 cells/μL) presents with fever, night sweats, and cervical lymphadenopathy. Lymph node aspirate shows AFB on ZN stain. Culture on LJ medium at 37°C grows slowly (8 weeks) with smooth, buff-colored colonies. On molecular drug susceptibility testing (GenoType MTBDRplus), no rpoB or katG mutations are detected. This is MOST consistent with:

• A Mycobacterium avium-intracellulare complex (MAC) infection ✓
• B Pan-sensitive Mycobacterium tuberculosis infection
• C Mycobacterium kansasii infection
• D Rifampicin-resistant TB (RR-TB) with a novel rpoB mutation

Explanation

Mycobacterium avium-intracellulare complex (MAC) causes disseminated infection in severely immunocompromised HIV patients (CD4 <50 cells/μL). MAC is AFB-positive on ZN stain and grows on LJ medium. GenoType MTBDRplus detects mutations in rpoB (rifampicin resistance) and katG/inhA (isoniazid resistance) specifically for M. tuberculosis complex — this assay will NOT detect these targets in MAC because the probe sequences are M. tuberculosis-specific; the test result appears negative/no mutation. Smooth buff-colored colonies slowly growing at 37°C are characteristic of MAC. M. kansasii produces photochromogenic colonies (yellow when light-exposed); pan-sensitive MTB would show a positive probe pattern on LPA.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.