A patient presents with facial flushing and laboratory findings showing markedly elevated hematocrit and erythrocyte mass; which diagnosis is most likely?

• A. Essential Thrombocytosis
• B. Polycythemia Vera
• C. Dehydration-Induced Polycythemia
• D. Polycythemia Due to Smoking

Answer: (B)

This question hinges on recognizing true polycythemia from a marrow-driven process. When both hematocrit and erythrocyte mass are markedly elevated, the red-cell volume is increased due to primary production, not just a loss of fluid. In polycythemia vera, a JAK2-activating mutation causes uncontrolled red-cell synthesis independent of erythropoietin, so erythropoietin levels are suppressed. The facial flushing (plethora) reflects the increased red-blood-cell mass and blood viscosity. Dehydration can raise the hematocrit by reducing plasma volume but does not increase the actual red-cell mass. Secondary polycythemia from smoking or hypoxia also elevates erythropoietin, driving red-cell production, which helps distinguish it from polycythemia vera. Thus, the most likely diagnosis is polycythemia vera.