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
Esophagus is constricted at 4 anatomic locations. Narrowest pa of esophagus lies at which of the following constriction? 
Select and explain. 
A. At the level of cricopharyngeal sphincter, B. At the crossing of aoic arch, C. At the crossing of left bronchus, D. At the level of opening in the diaphragm
A: At the level of cricopharyngeal sphincter
Answer: A: At the level of cricopharyngeal sphincter 
Explanation: Narrowest pa of esophagus is at its commencement at the cricopharyngeal sphincter which is 15cm from the incisor teeth. Other sites of esophagus where it is slightly constricted includes:Crossing by the aoic arch which is 22cm from the teeth.Crossing by the left principal bronchus which is 27cm from the teethAt the opening in the diaphragm which is 38cm from the teeth