**Core Concept**
Policymakers in hematology use specific diagnostic criteria, the Polycythemia Vera Study Group (PVSG) criteria, to identify polycythemia vera (PV), a type of myeloproliferative neoplasm characterized by excessive red blood cell production. The PVSG criteria help distinguish PV from other myeloproliferative disorders and secondary erythrocytosis.

**Why the Correct Answer is Right**
One of the major criteria for diagnosing polycythemia vera is the presence of a **JAK2 V617F mutation**. This genetic mutation leads to constitutive activation of the JAK-STAT signaling pathway, stimulating erythropoiesis (red blood cell production) independently of erythropoietin. The **JAK2 V617F mutation** is found in approximately 95% of PV patients and is considered a hallmark of the disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Elevated erythropoietin levels** are not a major criterion for PV. Elevated erythropoietin levels often indicate secondary erythrocytosis, which is caused by an external stimulus, such as renal disease or hypoxia.
* **Option B:** **Leukocytosis** is a common feature of PV but is not a major criterion for diagnosis. Leukocytosis can be seen in various myeloproliferative disorders and is not specific to PV.
* **Option C:** **Thrombocytosis** is another common feature of PV, but like leukocytosis, it is not a major criterion for diagnosis. Thrombocytosis can be seen in various myeloproliferative disorders and is not specific to PV.

**Clinical Pearl / High-Yield Fact**
In patients with suspected polycythemia vera, it is essential to check for the **JAK2 V617F mutation**. This genetic mutation is a major criterion for diagnosis and helps differentiate PV from other myeloproliferative disorders. The presence of a **JAK2 V617F mutation** also guides targeted therapy and monitoring.

**Correct Answer:** C.