Final answer:
Achalasia is the GI disease that has a bird's beak appearance on x-ray. It is a condition where the lower esophageal sphincter fails to relax properly, causing difficulty in swallowing.
Step-by-step explanation:
The GI disease that is commonly associated with a 'bird's beak' appearance on an x-ray is called achalasia. Achalasia is a condition in which the lower esophageal sphincter fails to relax properly, leading to difficulty in swallowing and the buildup of food in the esophagus. On x-ray, the narrowed lower esophageal sphincter resembles a bird's beak. Achalasia is diagnosed through various tests, such as barium swallow, esophageal manometry, and endoscopy.