Achalasia Bird-Beak Sign Barium

A 42-year-old woman with a 3-year history of progressive dysphagia to solids and liquids, nocturnal regurgitation of undigested food, and 8 kg weight loss presents for evaluation. Upper endoscopy excludes mechanical obstruction and malignancy but shows resistance at the gastroesophageal junction. Timed barium swallow esophagogram demonstrates a massively dilated esophagus with retained debris and a characteristic appearance at the gastroesophageal junction. The structure marked **A** in the diagram—smooth symmetric tapered narrowing extending 2–4 cm at the gastroesophageal junction producing the classic bird-beak sign—represents failure of which of the following mechanisms?