A 47-year-old man presents to your office complaining that for the past 6 months, food "feels like it gets stuck in my chest." The patient points to his midsternum as he describes the symptoms. He denies having trouble swallowing liquids, and he denies having odynophagia. His symptoms are sporadic and have not gotten worse recently. His weight is stable. On examination, the patient appears healthy. Results of laboratory studies are unremarkable.
On the basis of medical history, which of the following is the most likely cause of this patient's dysphagia?