A patient presents with increased erythropoietin, normal plasma volume, and increased RBC mass. What is the likely diagnosis?

The clinical scenario described indicates increased erythropoietin levels, normal plasma volume, and an increased red blood cell (RBC) mass. These findings are classic for secondary polycythemia.

In secondary polycythemia, the increase in erythropoietin is typically a response to chronic hypoxia or another condition that stimulates RBC production, such as a tumor producing erythropoietin. The key differentiator here is that, unlike primary polycythemia vera where erythropoietin levels are low due to a constitutive activation of the erythroid progenitors, this patient has elevated erythropoietin, reflecting an appropriate physiological response to a stimulus that leads to increased RBC production.

Normal plasma volume indicates that the increase in RBC mass is not a result of dehydration or a relative increase, which can complicate the assessment in conditions like polycythemia vera. Therefore, the described findings support a diagnosis of secondary polycythemia, where the body is responding to an external factor that leads to increased RBC production guided by elevated erythropoietin levels.