**Core Concept:**
Primary polycythemia (polycythemia vera) and secondary polycythemia are two major types of polycythemia, which is a condition characterized by an increased number of red blood cells and hemoglobin. Primary polycythemia is a myeloproliferative disorder while secondary polycythemia results from various underlying causes such as hypoxia, erythropoietin production, or erythropoietin resistance.

**Why the Correct Answer is Right:**
The correct answer is **D** - erythropoietin (EPO) measurement. Erythropoietin is a glycoprotein hormone produced by the kidneys in response to hypoxia (reduced oxygen levels) or anemia. In primary polycythemia, EPO levels are usually normal or suppressed despite elevated red blood cell count, as the disease itself is the cause of increased red blood cell production. On the other hand, in secondary polycythemia, EPO levels are elevated due to the underlying cause (e.g., hypoxia).

**Why Each Wrong Option is Incorrect:**
A. **Hemoglobin concentration:** While hemoglobin concentration may differ between primary and secondary polycythemia, this factor alone cannot differentiate them as it is not specific to either condition.

B. **Blood pressure:** Blood pressure can be elevated in both primary and secondary polycythemia due to increased red blood cell mass, but it is not specific to differentiate the two types.

C. **Platelet count:** Platelet count may be elevated in both conditions due to increased red blood cell mass, making it an unsuitable factor for differentiation.

**Clinical Pearl:**
A reliable way to differentiate primary polycythemia from secondary polycythemia is by measuring erythropoietin levels. A normal or suppressed EPO level in the presence of elevated hemoglobin and hematocrit levels is indicative of primary polycythemia (polycythemia vera). If erythropoietin levels are elevated, the diagnosis is likely secondary polycythemia, with hypoxia being the primary cause. This test is crucial in accurate diagnosis and treatment planning for these patients.