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 not a diagnostic feature of gallstone ileus on a plain abdominal radiograph? \nSelect and explain. \nA. Ectopic calcified gallstone, B. Stone < 2.5 cm size in the intestine, C. Signs of small intestinal obstruction, D. Gas in the biliary tree', 'role': 'user'}, {'content': "Answer: B \nExplanation: Characteristic feature of gallstone ileus is Rigler's triad :\n\nIncomplete or complete small bowel obstruction;\xa0\nGas in biliary tree;\xa0\nEctopic gallstone.", 'role': 'assistant'}]