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: Unknown
Zenker diveiculum occurs at: 
Select and explain. 
A. Upper esophageal spinchter, B. Lower esophageal sphincter, C. Both of the above, D. None of the above
A: Upper esophageal spinchter
Answer: A: Upper esophageal spinchter 
Explanation: Diveicula of the esophagus are mainly located above the LES (epiphrenic diveiculum) or the UES (pharyngoesophageal--or Zenker--diveiculum). They are both caused by abnormalities involving the LES or the UES, which result in protrusion of the mucosa and submucosa through the muscular layers