**Core Concept**
Stroke, also known as cerebrovascular accident (CVA), is a leading cause of morbidity and mortality worldwide. It occurs due to a sudden interruption in blood flow to the brain, leading to ischemia or hemorrhage. The underlying pathophysiology involves a complex interplay of vascular, neurological, and hemodynamic factors.

**Why the Correct Answer is Right**
The correct answer is patients with **atrial fibrillation (AF)**. AF increases the risk of stroke by promoting the formation of **thrombi** in the left atrium, which can then embolize to the brain. This is due to the irregular heart rhythm causing blood to pool and clot, increasing the risk of thromboembolic events.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while hypertension is a risk factor for stroke, it is not the most common underlying condition associated with stroke.
**Option B:** This option is incorrect because diabetes mellitus is a risk factor for stroke, but it is not the most direct cause of stroke.
**Option C:** This option is incorrect because while smoking is a risk factor for stroke, it is not the most common underlying condition associated with stroke.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that patients with AF are at increased risk of stroke, and anticoagulation therapy is often recommended to reduce this risk. The CHA2DS2-VASc score is a useful tool to assess the risk of stroke in patients with AF.

**Correct Answer: D. Patients with atrial fibrillation (AF).**