Polycythemia vera (PV) is distinguished from secondary polycythemia by which finding?
- A Low erythropoietin level with JAK2 V617F mutation positivity ✓
- B Hematocrit > 60% in men
- C Elevated EPO with normal oxygen saturation
- D Presence of splenomegaly on ultrasound
Correct answer: A. Low erythropoietin level with JAK2 V617F mutation positivity
Explanation
Polycythemia vera is a clonal myeloproliferative neoplasm driven by JAK2 V617F mutation (present in ~95% of cases) or exon 12 mutations. The key distinguishing laboratory feature from secondary polycythemia is a suppressed serum erythropoietin (EPO) level — because the autonomous erythroid proliferation does not require EPO. Secondary polycythemia (from hypoxia, EPO-secreting tumours) has elevated EPO. Splenomegaly can be seen in both PV and secondary causes. Hematocrit cutoffs alone are not diagnostic.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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