**Core Concept**
Polycythemia rubra vera (PRV) is a myeloproliferative disorder characterized by the excessive production of red blood cells, white blood cells, and platelets due to a somatic mutation in the JAK2 gene. This mutation leads to the activation of the JAK-STAT signaling pathway, resulting in the uncontrolled proliferation of hematopoietic cells.

**Why the Correct Answer is Right**
The correct features of PRV include:
- Increased red blood cell mass, leading to elevated hemoglobin levels and hematocrit.
- Leukocytosis, with an increased count of white blood cells, particularly neutrophils.
- Thrombocytosis, with an elevated platelet count.
- Splenomegaly, due to the sequestration of excess red blood cells.
- Pruritus, often associated with the elevated red blood cell mass.
- Risk of thrombotic events, such as stroke, myocardial infarction, and pulmonary embolism.

**Why Each Wrong Option is Incorrect**
**Option A:** PRV is often characterized by an elevated red blood cell mass, which can lead to symptoms such as headache, fatigue, and dizziness. This option is **not a feature of PRV**.
**Option B:** Leukocytosis and thrombocytosis are common features of PRV, but option B specifically mentions "lymphocytosis", which is not a characteristic of this condition. **Option B is incorrect**.
**Option C:** Splenomegaly is a common finding in PRV due to the sequestration of excess red blood cells. **Option C is correct**, so it is not the answer we are looking for.

**Clinical Pearl / High-Yield Fact**
The JAK2 V617F mutation is a somatic mutation that is present in the majority of PRV cases. This mutation leads to the activation of the JAK-STAT signaling pathway, resulting in the uncontrolled proliferation of hematopoietic cells. It is essential to recognize this mutation as a diagnostic criterion for PRV.

**Correct Answer:** B. Lymphocytosis.