**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 mutation in the JAK2 gene. This leads to an increased risk of thrombotic complications.

**Why the Correct Answer is Right**
The most significant complication of PRV is thrombosis, which occurs due to the increased viscosity of blood and the aggregation of platelets. The high red blood cell mass and platelet count lead to a hypercoagulable state, increasing the risk of venous and arterial thrombosis, including stroke, myocardial infarction, and pulmonary embolism. The increased risk of thrombosis is a direct consequence of the abnormal hematopoiesis in PRV.

**Why Each Wrong Option is Incorrect**
**Option A:** Hyperviscosity syndrome is a complication of PRV, but it is not the most significant complication. Hyperviscosity syndrome occurs when the blood becomes too thick, leading to symptoms such as headaches, confusion, and visual disturbances.
**Option B:** Leukostasis is a complication of acute myeloid leukemia, not PRV. Leukostasis occurs when the white blood cell count becomes too high, leading to symptoms such as respiratory distress and neurological impairment.
**Option C:** Splenomegaly is a common finding in PRV, but it is not the most significant complication. Splenomegaly occurs due to the sequestration of blood cells in the spleen.

**Clinical Pearl / High-Yield Fact**
PRV patients are at increased risk of thrombosis due to the hypercoagulable state, and anticoagulation therapy is often required to prevent thrombotic complications. Patients with PRV should be screened for thrombosis risk factors, including a history of previous thrombotic events.

**Correct Answer: D. Thrombosis**