This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Oesophagus, Subject name: ENT
[{'content': 'Narrowest part of esophagus is at? \nSelect and explain. \nA. Cricopharyngeal sphincter, B. At the Aortic constriction, C. At the crossing of left principal bronchus, D. At the diaphragmatic opening', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., Cricopharyngeal sphincter * The narrowest part of esophagus is its commencement at the cricopharyngeal sphincter.", 'role': 'assistant'}]