Reactivation pulmonary tuberculosis characteristically localizes to the apical and posterior segments of the upper lobes. The principal reason for this predilection is:

• A Poor lymphatic drainage of the lung apices
• B Deposition of inhaled bacilli by gravitational sedimentation
• C Higher oxygen tension favoring growth of the organism ✓
• D Reduced macrophage density in the upper lobe parenchyma

Explanation

Mycobacterium tuberculosis is an obligate aerobe whose growth is favored by high oxygen tension. The apical and posterior segments of the upper lobes have the highest ventilation to perfusion ratios in the upright lung, providing a well oxygenated niche for reactivation, whereas primary infection more often involves the better perfused mid and lower zones. Sedimentation of inhaled particles actually favors lower lobe deposition, which contradicts option B, making it incorrect.

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.