Which structure works to prevent reflux of gastric contents by contracting and relaxing?

The lower esophageal sphincter (LES) is a ring of smooth muscle located at the junction between the esophagus and the stomach. Its primary function is to prevent gastric contents, including acid, from flowing back into the esophagus. The LES contracts to keep the stomach contents contained and relaxes to allow food to enter the stomach during swallowing. Dysfunction of the LES can lead to gastroesophageal reflux disease (GERD). The esophageal muscularis helps move food, the upper esophageal sphincter controls entry into the esophagus, and the angle of His is an anatomical feature rather than a muscular valve. The NEX Science exam frequently tests digestive system physiology and reflux prevention mechanisms.