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
A 59-year-old male came with Hb 18.0 gm/dl on three occasions. The resident doctor wants to exclude Polycythemia Vera. Which of the following is the most relevant investigation : 
Select and explain. 
A. Hematocrit, B. Total leukocyte count, C. Red cell mass, D. Reticulocyte count
C: Red cell mass
Answer: C: Red cell mass 
Explanation: Answer is C (Red cell mass): When confronted with an elevated Haemoglobin or Haematocrit the first step should be to determine whether this increase is due to absolute erythrocytosis or relative erythrocytosis. Absolute erythrocytosis Relative/ spurious erythrocytosis Apparent increase in red cell mass due to reduction in plasma volume True increase in red cell mass `Red cell mass' is the investigation of choice to determine such distinction. Once 'red cell mass' determination confirms absolute erythrocytosis, a diagnosis of polycythemia vera can be established by demonstrating absence of elevation in serum erythropoetin levels.