This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, 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: Left main stem bronchus crosses the esophagus.