A 52-year-old man for the last 3 years has been suffering from difficult swallowing of solid food, burning retrosternal pain that aggravated during eating, loss of body mass, and occasional vomiting with undigested food. Esophageal X-ray shows S-shaped deformation of the esophagus and its dilation; at the cardiac orifice the esophagus is constricted: esophageal mucosa is smooth, without signs of peristalsis. Make the provisional diagnosis