Reactivation tuberculosis in adults characteristically involves the lung apex. This predilection for apical segments is best explained by:

• A Poorly developed apical lymphatics allowing bacilli to persist unchecked
• B Direct hematogenous seeding of the apex from a distant focus
• C High oxygen tension favoring growth of the strictly aerobic bacillus ✓
• D Reduced alveolar macrophage density at the lung apices

Explanation

Mycobacterium tuberculosis is an obligate aerobe, and the apical and posterior segments of the upper lobes have the highest ventilation to perfusion ratios and oxygen tensions in the lung, providing the environment where dormant bacilli resume growth in secondary tuberculosis. Primary infection, by contrast, occurs in the subpleural parenchyma of the mid and lower lobes near the hilum, forming the Ghon complex, which rules out option B as the mechanism of reactivation site selection.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.