**Core Concept**
Postoperative thromboembolism, particularly after pelvic surgery, is a significant concern due to prolonged immobilization and increased venous stasis. The veins are more susceptible to thrombosis due to their higher pressure and slower blood flow compared to arteries.

**Why the Correct Answer is Right**
The development of thromboembolism after pelvic surgery is often attributed to the formation of deep vein thrombosis (DVT) in the lower extremities, particularly in the deep veins of the calf and thigh. This is due to the increased pressure and venous stasis caused by prolonged bed rest and decreased mobility, which leads to activation of the coagulation cascade and platelet aggregation. The resulting thrombi can then break loose and travel to the lungs, resulting in pulmonary embolism (PE).

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while arterial thromboembolism can occur, it is relatively rare in the context of pelvic surgery and postoperative care.
* **Option B:** This option is incorrect because while the arteries are more susceptible to atherosclerotic disease, they are less commonly involved in postoperative thromboembolism.
* **Option C:** This option is incorrect because while the pelvic veins can be involved in postoperative thromboembolism, the primary concern is the development of DVT in the lower extremities.

**Clinical Pearl / High-Yield Fact**
The American College of Chest Physicians (ACCP) recommends the use of mechanical prophylaxis (e.g., compression stockings, intermittent pneumatic compression) and/or pharmacological prophylaxis (e.g., low molecular weight heparin, warfarin) to prevent DVT and PE in patients undergoing pelvic surgery.

**Correct Answer:** D.