Which structure prevents air from entering the esophagus during breathing?

• A. LES
• B. Pyloric sphincter
• C. UES
• D. Cardiac sphincter

Answer: (C)

The key idea is that the upper esophageal sphincter forms the gate between the pharynx and the esophagus and stays closed during normal breathing. This keeps air from being directed into the esophagus as you inhale. It only relaxes briefly to allow a swallow, after which it tightens again to maintain separation between the airway and the food pipe.

The other options either regulate movement into the stomach or are just another name for the lower esophageal sphincter, which sits at the junction with the stomach and prevents reflux, not air entry during breathing. The pyloric sphincter sits between the stomach and the duodenum and has nothing to do with protecting the esophagus during respiration.