question: Which sphincter is located at the stomach end of the esophagus and opens to allow the bolus to enter the stomach?
distractor3: urethral sphincter
distractor1: pupillary sphincter
distractor2: pyloric sphincter
correct_answer: gastro-esophageal sphincter
support: A ring-like muscle called a sphincter forms valves in the digestive system. The gastro-esophageal sphincter is located at the stomach end of the esophagus. In response to swallowing and the pressure exerted by the bolus of food, this sphincter opens, and the bolus enters the stomach. When there is no swallowing action, this sphincter is shut and prevents the contents of the stomach from traveling up the esophagus. Many animals have a true sphincter; however, in humans, there is no true.