The lower esophageal sphincter (LES) is described as which of the following structures?

• A. A muscular ring that closes off the esophagus from the stomach
• B. A valve that opens only during sleep
• C. A structure that aids in bolus grinding
• D. A cartilage ring in the respiratory tract

Answer: (A)

The main idea here is understanding what the lower esophageal sphincter does at the gastroesophageal junction. It is a thickened ring of smooth muscle that remains tonically contracted to close off the esophagus from the stomach, acting like a valve to prevent gastric reflux. During swallowing, this ring relaxes in a coordinated swallow so the bolus can pass into the stomach, and then it tightens again to restore closure. It is not something that opens only during sleep, and it does not function to grind food—that’s the role of mechanical digestion in the mouth and stomach. It also isn’t a cartilage ring in the respiratory tract. So, describing it as a muscular ring that closes off the esophagus from the stomach best captures its anatomical and functional role.