This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Haematology, Subject name: Pathology
In which of the following conditions that cause polycythemia is the serum erythropoietin extremely low - 
Select and explain. 
A. Dehydration, B. Renal cell carcinoma, C. Renal cell carcinoma, D. Polycythemia vera
D: Polycythemia vera
Answer: D: Polycythemia vera 
Explanation: In polycythemia vera unlike secondary polycythemia erythropoietin levels in serum and urine are reduced. Reference textbook of Pathology 7th edition author Harsha Mohan page number 339