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 ulcerative colitis is: \nSelect and explain. \nA. Pseudopolyp formation, B. Loss of haustrations, C. Tubular colon, D. Increased retrorectal space', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. Loss of haustrations', 'role': 'assistant'}]