A 74-year-old male complains of bad breath, regurgitation of food during his sleep, and weight loss. For several months he has also had trouble swallowing solids. He denies fever, chills, and nausea. The patient does not smoke cigarettes and does not drink alcohol. Barium swallow study with fluoroscopy is obtained, one radiograph among the series is shown below. Which of the following is the most likely underlying cause of the patient's condition