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
Which of the following is not a cause of secondary polycythemia? 
Select and explain. 
A. High Altitude, B. Myeloproliferative changes, C. Pheochromocytoma, D. Cerebellar hemangioblastoma
B: Myeloproliferative changes
Answer: B: Myeloproliferative changes 
Explanation: None