**Core Concept**
In the context of ischemic stroke, embolism occurs when a blood clot or other material breaks loose from the heart or a large artery and travels to the brain, blocking a cerebral artery and causing a stroke. The most common cause of embolic stroke is related to the heart, particularly in the setting of atrial fibrillation.

**Why the Correct Answer is Right**
The correct answer is related to the heart's role in generating emboli. Atrial fibrillation (AF) is a common cardiac arrhythmia characterized by rapid and irregular heartbeats, leading to the formation of blood clots in the left atrium. These clots can then embolize to the brain, causing an ischemic stroke. The risk of stroke is significantly higher in patients with AF, especially those with additional risk factors such as hypertension, diabetes, and previous stroke or transient ischemic attack.

**Why Each Wrong Option is Incorrect**
* **Option A:** While cardioembolism is the most common cause of embolic stroke, other sources like carotid artery stenosis or dissection are less common but still potential causes.
* **Option B:** Intraplaque hemorrhage within a carotid atherosclerotic plaque is a possible source of emboli, but it is not the most common cause of embolic stroke.
* **Option D:** While vasculitis can cause stroke, it is relatively rare compared to embolic causes like atrial fibrillation.

**Clinical Pearl / High-Yield Fact**
Atrial fibrillation is a significant risk factor for embolic stroke, and patients with AF should be anticoagulated to reduce the risk of stroke. The CHA2DS2-VASc score is a useful tool to assess stroke risk in patients with AF.

**Correct Answer:** C. Atrial fibrillation