**Core Concept**
The most common cause of embolic stroke is related to the formation of blood clots in the heart, particularly in the left atrium, which can break loose and travel to the brain, causing a blockage in a cerebral artery.

**Why the Correct Answer is Right**
Embolic strokes occur when a blood clot or other debris forms in a distant part of the circulation and travels to the brain, where it blocks a cerebral artery. The most common source of these emboli is the left atrium, particularly in the context of atrial fibrillation. In atrial fibrillation, the irregular heart rhythm can lead to the formation of blood clots in the left atrium, which can then break loose and embolize to the brain. This is often referred to as a cardioembolic stroke.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while atherosclerosis is a common cause of ischemic stroke, it is not typically associated with embolic stroke. Atherosclerosis involves the buildup of plaque in the walls of arteries, leading to a blockage of blood flow, but it does not typically lead to the formation of emboli.
**Option B:** This option is incorrect because while hypertension is a major risk factor for stroke, it is not a direct cause of embolic stroke. Hypertension can lead to damage to blood vessels and increase the risk of atherosclerosis, but it is not typically associated with the formation of emboli.
**Option C:** This option is incorrect because while vasculitis is a rare cause of stroke, it is not typically associated with embolic stroke. Vasculitis involves inflammation of the blood vessels, which can lead to blockage or damage to the vessels, but it is not typically associated with the formation of emboli.

**Clinical Pearl / High-Yield Fact**
It's essential to note that embolic strokes often have a more sudden onset than ischemic strokes, and patients may experience a sudden loss of neurological function, such as weakness or numbness on one side of the body, difficulty speaking, or vision changes.

**Correct Answer: A. Cardiac source of embolism.