This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Radiology
[{'content': "First radiologic sign of Crohn's disease in the terminal ileum is \nSelect and explain. \nA. Aphthoid ulceration, B. Pseudosacculation, C. Cobble stone pattern, D. Thickening of bowel wall", 'role': 'user'}, {'content': 'Answer: A \nExplanation: .', 'role': 'assistant'}]