**Core Concept**
Cerebrovascular accident (CVA), commonly known as a stroke, is a condition where the blood supply to the brain is interrupted, leading to brain tissue damage. The most common cause of CVA is atherosclerosis, a condition characterized by the buildup of plaque in the arterial walls.

**Why the Correct Answer is Right**
Atherosclerosis leads to the narrowing and hardening of arteries, which increases the risk of thrombosis or embolism. This reduces blood flow to the brain, resulting in ischemia and subsequent tissue damage. The increased risk of thrombosis is due to the activation of platelets and the formation of blood clots on the surface of the plaque. The narrowing of arteries also increases the risk of rupture, leading to hemorrhagic stroke. The most common location for atherosclerosis-related CVA is the anterior circulation of the Circle of Willis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hypertension is a risk factor for atherosclerosis, but it is not the most common cause of CVA. While hypertension can lead to stroke, it is often a contributing factor rather than the primary cause.
**Option B:** This option is incorrect because cardiac embolism is a common cause of CVA, but it is not the most common cause. Cardiac sources of embolism, such as atrial fibrillation, can lead to stroke, but they are less common than atherosclerosis-related CVA.
**Option C:** This option is incorrect because vasculitis is a rare cause of CVA. While vasculitis can lead to stroke, it is not a common cause.

**Clinical Pearl / High-Yield Fact**
Atherosclerosis is a major risk factor for CVA, and its prevention is key to reducing the risk of stroke. The management of atherosclerosis-related CVA involves the use of antiplatelet agents, anticoagulants, and statins to reduce the risk of further thrombosis and improve blood flow to the brain.

**Correct Answer: A. Atherosclerosis.**