PA25.4 | Pulmonary Tuberculosis — Summary & Reflection

REFLECT

You have just traced tuberculosis from a single inhaled bacillus to a destroyed lung and disseminated systemic disease. Before moving on, reflect on these questions — write a short answer (3–5 sentences each):

1. Why does tuberculosis cause caseous necrosis rather than the liquefactive or coagulative necrosis seen in other infections? What immune mechanism is responsible?

1. A 50-year-old patient with rheumatoid arthritis is about to start anti-TNF-α therapy. Explain, using the pathogenesis of TB, why she must be screened for latent TB before starting treatment.

1. From a pathological standpoint, what makes a TB cavity so dangerous — both for the patient and for public health?

Think carefully, then compare your answers with the teaching pearls in the faculty notes.

KEY TAKEAWAYS

Pulmonary Tuberculosis — Key Takeaways:

• Organism: M. tuberculosis — acid-fast bacillus (ZN stain: beaded red rods), mycolic acid cell wall blocks phagolysosome fusion.

• Pathogenesis: Type IV (delayed-type) hypersensitivity + Th1-driven CMI → epithelioid granuloma with central caseous necrosis → containment (latency) or progressive disease.

• Primary TB: Ghon focus (lower part of upper lobe, subpleural) + hilar lymph node = Ghon complex → usually heals, calcifies; progressive primary in immunocompromised.

• Secondary TB: Reactivation in apices → caseous consolidation → cavity → the paradigm of adult pulmonary TB.

• Micro hallmarks: Caseating epithelioid granuloma with Langhans giant cells (horseshoe nuclei) + lymphocytic cuff + caseous centre; ZN-positive AFB.

• Spread routes: Haematogenous → miliary TB (lungs + meninges + spine + kidneys + adrenals); endobronchial → tuberculous bronchopneumonia; lymphatic → lymphadenitis.

• Complications: Cavitation, Rasmussen aneurysm + haemoptysis, empyema, fibrosis, bronchiectasis, cor pulmonale, amyloidosis, extrapulmonary dissemination.

• Diagnosis: Sputum ZN smear + GeneXpert CBNAAT (RIF resistance in 2 hours) + culture (gold standard) + Mantoux/IGRA + HRCT.

• Drug resistance: MDR = resistant to INH + RIF; XDR = MDR + fluoroquinolone + injectable agent. India has the highest absolute MDR-TB burden globally.