**Core Concept**
PET scans are used to evaluate cardiac function by assessing myocardial blood flow and metabolism. Pharmacological stressors are used to induce stress on the heart, allowing for evaluation of cardiac function under stress conditions.

**Why the Correct Answer is Right**
Adenosine is a potent vasodilator that increases blood flow to the heart, increasing myocardial oxygen demand. This increase in oxygen demand is used as a stressor to evaluate cardiac function. Adenosine works by binding to A2A receptors on vascular smooth muscle cells, causing vasodilation and an increase in coronary blood flow. This increase in blood flow is used to evaluate cardiac function, particularly in patients who are unable to undergo exercise stress testing.

**Why Each Wrong Option is Incorrect**
**Option A:** Dobutamine is a beta-adrenergic agonist that increases heart rate and contractility, but it is not typically used as a pharmacological stressor in PET scans.
**Option B:** Atropine is an anticholinergic agent that increases heart rate by blocking the vagus nerve, but it is not typically used as a pharmacological stressor in PET scans.
**Option C:** Nitrates are vasodilators that decrease blood pressure and coronary blood flow, which is the opposite of what is desired in a pharmacological stressor.

**Clinical Pearl / High-Yield Fact**
Adenosine is often used as a pharmacological stressor in PET scans because it is a potent vasodilator that increases coronary blood flow without increasing myocardial oxygen demand.

**Correct Answer:** C. Adenosine