Paper I — 2025 August (Supplementary) (2010 Scheme) — Q3
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Paper I
2025 August (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Laboratory diagnosis of pulmonary tuberculosis
Q35 marksShort Essays
Answer
Laboratory diagnosis of pulmonary tuberculosis (caused by Mycobacterium tuberculosis):
Sputum smear microscopy — Ziehl-Neelsen staining for acid-fast bacilli on at least two samples; fluorescence (auramine-rhodamine) microscopy is more sensitive for screening.
Culture — the gold standard. Lowenstein-Jensen medium (egg-based, malachite-green selective) gives characteristic rough, buff-coloured “cauliflower” colonies but is slow (2–8 weeks); liquid culture (BACTEC MGIT) is faster and permits drug-susceptibility testing.
Molecular methods — CBNAAT/GeneXpert MTB-RIF, rapid PCR-based test detecting M. tuberculosis DNA and rifampicin resistance within ~2 hours; line-probe assays.
Tuberculin skin test (Mantoux) — indicates prior sensitization/exposure; can be falsely positive after BCG vaccination.
Interferon-Gamma Release Assays (IGRAs) — more specific than TST, unaffected by BCG vaccination.
Radiology — chest X-ray showing apical infiltrates, cavitation, or fibrosis.
Histopathology — caseating granuloma with Langhans giant cells on biopsy, when tissue is available.
Sputum smear microscopy remains the primary rapid, low-cost tool, with CBNAAT increasingly used as the preferred initial test where available, and culture reserved for confirmation and drug-susceptibility testing.
