Achalasia Manometry and Endoscopic Findings

A 42-year-old man presents with a 6-month history of progressive dysphagia to both solids and liquids, regurgitation of undigested food, and retrosternal chest pain. Upper endoscopy reveals a dilated esophagus with retained food debris and a tightly closed lower esophageal sphincter (LES) that requires gentle pressure to pass. The structure marked **A** in the diagram shows these characteristic findings. High-resolution esophageal manometry confirms failed LES relaxation with 100% failed peristalsis and no esophageal pressurization. Which of the following pathophysiologic mechanisms BEST explains the manometric findings in this patient?