This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
[{'content': "Which of the following is the earliest sign of ulcerative colitis on double-contrast barium enema study - \nSelect and explain. \nA. Mucosal granularity, B. Loss of haustrations, C. 'Collar - button' ulcer, D. Lead pipe colon", 'role': 'user'}, {'content': "Answer: A \nExplanation: Earliest radiological change of UC on barium enema is blurring of the mucosal lining and a fine granularity when mucosa is seen enface due to edema.\nEarliest radiological change of Crohn's disease\xa0→\xa0Aphthous lesions (Bull's eye or Target lesions).", 'role': 'assistant'}]