**Core Concept**
Intermittent claudication is a symptom characterized by muscle pain or cramping in the legs that occurs during walking or exercise, relieved by rest. It is primarily caused by a reduction in blood flow to the muscles, resulting from atherosclerosis of the lower extremity arteries.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of intermittent claudication. Atherosclerosis is the buildup of plaque in the arterial walls, leading to a narrowing of the lumen and a reduction in blood flow. This reduction in blood flow to the muscles of the lower extremities results in ischemia, causing pain during exercise. The most common arteries affected are the femoral and popliteal arteries, which supply blood to the muscles of the lower extremities.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because peripheral neuropathy can cause pain in the legs, but it does not cause intermittent claudication. Peripheral neuropathy is related to damage to the nerves, not a reduction in blood flow.
**Option B:** This option is incorrect because vasculitis is an inflammatory condition of the blood vessels, which can cause a range of symptoms, but it is not the primary cause of intermittent claudication.
**Option C:** This option is incorrect because venous insufficiency can cause pain in the legs, but it does not cause intermittent claudication. Venous insufficiency is related to problems with the veins, not a reduction in blood flow.

**Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that patients with intermittent claudication often have a history of smoking, hypertension, and hyperlipidemia, which are risk factors for atherosclerosis.

**Correct Answer: D. Atherosclerosis**