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: Surgery
[{'content': 'Deep vein thrombosis is seen in all except one - \nSelect and explain. \nA. Prolonged immobilization, B. Subungual melanoma, C. Pregnancy, D. Major surgical operation.', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]