Which condition represents true increase in RBC mass independent of plasma volume?

• A. Relative polycythemia
• B. Polycythemia Vera
• C. Primary polycythemia
• D. Secondary polycythemia

Answer: (B)

The key idea is distinguishing true, autonomous red cell production from changes in plasma volume. Polycythemia vera represents a true increase in red blood cell mass produced by a clonal expansion of erythroid precursors in the bone marrow. It is an intrinsic, autonomous process (often with a JAK2 mutation) that raises RBC mass regardless of plasma volume, and erythropoietin levels are typically suppressed. In contrast, relative polycythemia is just dilutional—plasma volume is reduced, so hematocrit appears high even though total RBC mass hasn’t increased. Secondary polycythemia involves increased RBC production in response to elevated erythropoietin from hypoxia or tumors; here the rise in RBC mass tracks the stimulus rather than being an autonomous, plasma-volume–independent expansion.