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
[{'content': 'Essential WHO criteria for polycythemia vera? \nSelect and explain. \nA. Tyrosine kinase JAK2 V617F and other mutations, B. Low levels of erythropoietin levels, C. Thrombocytosis, D. Increased MCV', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]