A 45-year-old man presents with a 3-year history of progressive dysphagia to both solids and liquids, regurgitation of undigested food at night, and a 12 kg weight loss. Upper endoscopy reveals a dilated esophagus with retained food debris and a tight lower esophageal sphincter (LES) that yields to gentle endoscope pressure, as shown in the structure marked **A** in the diagram. High-resolution manometry confirms absent esophageal contractility with elevated integrated relaxation pressure (IRP 18 mmHg). Which of the following pathophysiological mechanisms best explains the findings in structure **A**?