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: Physiology
[{'content': 'Which of the following is not a cause of secondary polycythemia? \nSelect and explain. \nA. High Altitude, B. Myeloproliferative changes, C. Pheochromocytoma, D. Cerebellar hemangioblastoma', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]