This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Myeloproliferative Disorders, Subject name: Pathology
A 60-year-old man is referred for evaluation of marked erythrocytosis and splenomegaly. Laboratory studies confirm an elevated red blood cell count and additionally demonstrate a moderate increase in circulating granulocytes and platelets. Oxygen saturation studies are normal, and isotopic studies reveal an increase in total red cell mass. Which of the following is characteristic of this disorder? 
Select and explain. 
A. Frequent association with thrombosis or hemorrhagic phenomena, B. Increased erythropoietin concentration, C. Manifestation of Cushing syndrome, D. Most often secondary to hypoxia
A: Frequent association with thrombosis or hemorrhagic phenomena
Answer: A: Frequent association with thrombosis or hemorrhagic phenomena 
Explanation: The diagnosis is polycythemia vera (primary polycythemia), one of the myeloproliferative syndromes. The disorder is characterized by prominent erythrocytosis, moderate granulocytosis, and thrombocytosis. Because of hyperviscosity and sludging of blood, there is a frequent association with thrombosis or hemorrhagic phenomena. Marked splenomegaly and decreased erythropoietin are other classic characteristics.