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: Medicine
All of the following are the causes of relative polychthemia except - 
Select and explain. 
A. Dehydration, B. Dengue haemorrhagic fever, C. Gaisbock syndrome, D. High altitude
D: High altitude
Answer: D: High altitude 
Explanation: None