**Core Concept**
Polycythemia vera is a myeloproliferative disorder characterized by the excessive production of red blood cells, white blood cells, and platelets. This condition is caused by a mutation in the JAK2 gene, leading to the activation of the JAK-STAT signaling pathway, which results in the overproduction of blood cells.

**Why the Correct Answer is Right**
In polycythemia vera, the levels of red blood cells (RBCs), white blood cells (WBCs), and platelets are all elevated due to the increased proliferation and survival of these cells. The JAK2 mutation leads to the activation of the JAK-STAT pathway, which promotes the transcription of genes involved in cell proliferation and survival.

**Why Each Wrong Option is Incorrect**
* **Option A:** Elevated RBC mass is a hallmark of polycythemia vera, so this option is likely incorrect.
* **Option B:** Elevated WBC count is also a common finding in polycythemia vera, so this option is likely incorrect.
* **Option C:** Elevated platelet count is a characteristic feature of polycythemia vera, so this option is likely incorrect.

**Clinical Pearl / High-Yield Fact**
In polycythemia vera, patients may also develop pruritus, which is thought to be caused by the release of histamine from mast cells. This symptom can be a valuable clue in the diagnosis of this condition.

**Correct Answer: D.**