**Core Concept**
Polycythemia vera (PV) is a myeloproliferative disorder characterized by the clonal expansion of erythroid, granulocytic, and megakaryocytic progenitor cells, leading to an overproduction of red blood cells, white blood cells, and platelets. This results in an increased risk of thrombotic complications due to the increased viscosity of blood.

**Why the Correct Answer is Right**
The correct answer is related to the increased risk of thrombosis in patients with PV. This is due to the increased blood viscosity, which can lead to the formation of blood clots in various parts of the body, including the arteries and veins. The increased platelet count also contributes to the risk of thrombosis. The high levels of erythropoietin in PV can also stimulate the production of von Willebrand factor, which is a key factor in the formation of blood clots.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while hypertension is a common complication of PV, it is not the most common complication.
**Option B:** This option is incorrect because while infections are a concern in PV patients, they are not the most common complication.
**Option C:** This option is incorrect because while bleeding is a risk in PV patients, it is not the most common complication.

**Clinical Pearl / High-Yield Fact**
It is essential to consider the risk of thrombosis in patients with PV when assessing their overall risk and developing a treatment plan. Anticoagulation therapy may be necessary in some cases to reduce the risk of thrombotic complications.

**Correct Answer: D. Thrombosis**