A 22-year-old woman has had multiple episodes of aspiration of food associated with difficulty swallowing during the past year. On auscultation of her chest, crackles are heard at the base of the right lung. A barium swallow shows marked esophageal dilation above the level of the lower esophageal sphincter. A biopsy specimen from the lower esophagus shows an absence of the myenteric ganglia. What is the most likely diagnosis?