A 30-year-old man was referred to a surgeon because he has been having increasing difficulty swallowing both solids and liquids. Physical examination of the patient is noncontributory. The patient swallows a barium solution, with continuous fluoroscopy (X-ray recording) is shown below. Esophageal manometry shows incomplete relaxation of the lower esophageal sphincter in response to swallowing, high resting lower esophageal pressure, and absent esophageal peristalsis. The manometry and barium swallow studies most strongly suppo which of the following diagnoses?