In erect posture commonest site of foreign body in bronchus -
From Schwart, 7/e→ "The most common anatomic location for a foreign body is the right main stem bronchus or the rt. lower lobe."
From Pretest (Pretest has quoted Harrison, 14/e, p 994)
" The right main stem bronchus is wider, shorter and vertically placed, and therefore the posterior segments of the right upper lobe (if the pt. aspirated while supine) are anatomically susceptible to aspiration pneumonia. The superior segments of the right lower lobe and left lower lobe (if the pt. supine) are also susceptible to aspiration pneumonia. These three segments are often referred to as the aspiration segments of the lung. The basilar segments of both lungs are susceptible to aspiration if the pts. aspirates while erect or sitting up."


Inference : Segments involved

Aspiration in Supine Position :

RL upper lobe - posterior segment
Rt. lower lobe - superior segment
Lt. lower lobe - superior segment

Aspiration in sitting or erection position

Rt. basilar segments (of lower lobe).
Right posterior basal
Right anterior basal
Lateral basal
Medial basal