**Core Concept**
Myocardial infarction (MI), commonly known as a heart attack, is the most common cause of sudden cardiac death. It occurs when the blood flow to the heart is severely reduced, leading to ischemia and subsequent necrosis of cardiac tissue.

**Why the Correct Answer is Right**
The most common cause of sudden death is myocardial infarction, which is often due to a blockage in one of the coronary arteries. This blockage is usually caused by a blood clot that forms on a pre-existing atherosclerotic plaque. The reduction in blood flow leads to a cascade of events, including ischemia, acidosis, and eventually, cardiac necrosis. The heart muscle becomes unable to pump effectively, leading to sudden cardiac death.

**Why Each Wrong Option is Incorrect**
**Option A:** While hypertension is a major risk factor for cardiovascular disease, it is not the most common cause of sudden death.
**Option B:** Stroke is a leading cause of death, but it is not the most common cause of sudden cardiac death.
**Option C:** Pulmonary embolism can cause sudden death, but it is less common than myocardial infarction.

**Clinical Pearl / High-Yield Fact**
The most common time for myocardial infarction to occur is between 6 am and 10 am, often referred to as the "morning peak." This is due to increased sympathetic activity and heightened blood pressure during this time period, which can exacerbate atherosclerosis and increase the risk of clot formation.

**Correct Answer:** A. Myocardial infarction.