This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Thorax, Subject name: Anatomy
The normal narrowing in the middle of the oesophagus is caused by 
Select and explain. 
A. Azygos vein, B. Hemiazygos vein, C. Right main stem bronchus, D. Left main stem bronchus
D: Left main stem bronchus
Answer: D: Left main stem bronchus 
Explanation: Middle constriction of oesophagus is due to left bronchus .27.5cm/11 inch from insisor teeth. BD CHAURASIA&;S HUMAN ANATOMY VOLUME 1, 6TH Edition . page no 282, fig 20.7