Final answer:
A 35-year-old man presenting with difficulty swallowing, chest pain, and regurgitation likely has Achalasia, a condition most commonly caused by a primary idiopathic process involving esophageal nerve degeneration.
Step-by-step explanation:
The condition described by the symptoms of difficulty swallowing (dysphagia), chest pain, regurgitation of undigested food, absence of a fundic air shadow on X-ray, and a beak-shaped lower end of the esophagus is suggestive of Achalasia. The barium studies typically show a dilated esophagus with a beak-shaped lower end due to the failure of the lower esophageal sphincter to relax. The most common cause for Achalasia is a primary idiopathic process, where the etiology is not fully understood, but is thought to involve the degeneration of nerves in the esophagus.