Where is TB most commonly found in the lungs and why?

Where is TB most commonly found in the lungs and why?

• A. Lower lobes due to greater perfusion.
• B. Upper lobes, apical and posterior segments, due to higher oxygen tension and lower lymphatic flow.
• C. Middle lobes because of higher lymphatic flow.
• D. Random distribution in all lobes.

Answer: (B)

TB is an aerobic organism, so it tends to establish in areas of the lung with higher oxygen levels. The apical and posterior segments of the upper lobes have the highest oxygen tension due to their ventilation–perfusion pattern (a higher V/Q ratio), and they also have relatively lower lymphatic clearance. This combination makes those upper-lobe regions most favorable for Mycobacterium tuberculosis to grow and form the characteristic cavitary lesions seen in reactivation TB. Lower lobes have more perfusion and lower oxygen tension, which is less conducive to TB growth, and middle lobes don’t have the same high-oxygen environment. So the typical site is the apical/posterior segments of the upper lobes because of higher oxygen tension and lower lymphatic flow.

TB is an aerobic organism, so it tends to establish in areas of the lung with higher oxygen levels. The apical and posterior segments of the upper lobes have the highest oxygen tension due to their ventilation–perfusion pattern (a higher V/Q ratio), and they also have relatively lower lymphatic clearance. This combination makes those upper-lobe regions most favorable for Mycobacterium tuberculosis to grow and form the characteristic cavitary lesions seen in reactivation TB. Lower lobes have more perfusion and lower oxygen tension, which is less conducive to TB growth, and middle lobes don’t have the same high-oxygen environment. So the typical site is the apical/posterior segments of the upper lobes because of higher oxygen tension and lower lymphatic flow.