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
A 32 year old high altitude mountaineer is observed to have a hematocrit of 70%. Which of the following represents the most likely cause/explanation 
Select and explain. 
A. Polycythemia with increased red cell mass, B. Relative polycythemia due to dehydration, C. Polycythemia due to hemoconcentration, D. Polycythemia with high altitude pulmonary edema
A: Polycythemia with increased red cell mass
Answer: A: Polycythemia with increased red cell mass 
Explanation: Erythropoietin secretion increases promptly on ascent to high altitude. The increase in circulating red blood cells triggered by the erythropoietin  begins in  2-3 days and is sustained as long as the individual remains at high altitude.