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': "First radiological sign of Crohn's disease in terminal ileum is: \nSelect and explain. \nA. Aphthoid ulceration, B. Pseudo sacculation, C. Cobblestone pattern, D. Thickening of bowel wall", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. Aphthoid ulceration', 'role': 'assistant'}]