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
[{'content': 'The normal narrowing in the middle of the oesophagus is caused by - \nSelect and explain. \nA. Azygos vein, B. Hemiazygos vein, C. Right main stem bronchus, D. Left main stem bronchus.', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Left main stem bronchus crosses the esophagus.', 'role': 'assistant'}]