This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Small & Large Intestine, Subject name: Surgery
[{'content': 'A 74-year-old woman complains of vomiting and intermittent colicky abdominal pain. X- rays reveal fluid levels and air in the biliary tree. What is the likely cause? \nSelect and explain. \nA. Abdominal adhesions, B. Gallstone ileus, C. Carcinoma of the right colon, D. Abdominal lymphosarcoma', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Gallstone ileus results in "tumbling" intestinal obstruction due to the intermittent nature of the condition. Previous choledochoduodenostomy could give air in the biliary tree but not obstruction.', 'role': 'assistant'}]