Zenker Diverticulum Pharyngoesophageal

A 71-year-old man presents with a 2-year history of progressive cervical dysphagia, regurgitation of undigested food material several hours after meals, halitosis, and nocturnal cough. He reports using the Boyce maneuver (pressing on the left neck) to empty his throat after meals. Lateral barium swallow esophagogram shows the structure marked **A** — a posterior outpouching arising from the hypopharynx just above the cricopharyngeus muscle at Killian's dehiscence. Which of the following best describes the pathophysiological mechanism underlying the formation of the structure marked **A**?