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
Most significant complication of polycythemia rubra vera is: 
Select and explain. 
A. Transformation into leukemia, B. Secondary bone marrow failure, C. Hypersplenism, D. Thrombosis
D: Thrombosis
Answer: D: Thrombosis 
Explanation: Polycythemia rubra vera It is an indolent disorder Thrombosis is the most significant complication Maintenance of the hemoglobin level at 140 g/L (14 g/dL; hematocrit Phlebotomy :serves to help this aim Ref: Harrison, E-18,P-903