The upper esophageal sphincter (UES) is a critical muscular valve located at the junction of the pharynx and esophagus, playing a crucial role in swallowing by preventing the entry of air into the esophagus and the backflow of esophageal contents into the pharynx. Dysfunction of the UES can lead to swallowing disorders and is often evaluated in the context of conditions like dysphagia and gastroesophageal reflux disease.