<prompt> Zenker diveiculum occurs at:Upper esophageal spinchterLower esophageal sphincterBoth of the aboveNone of the above  <response>: 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 