**Core Concept**
Obligate coronary vasodilators are substances that cause a significant increase in coronary blood flow by dilating the coronary arteries. This occurs in response to increased myocardial oxygen demand or to maintain a constant coronary blood flow despite changes in systemic blood pressure.

**Why the Correct Answer is Right**
The correct answer is a substance that does not cause significant vasodilation in the coronary arteries. In contrast, obligate coronary vasodilators include adenosine, which acts on A2 receptors to cause vasodilation, and nitroglycerin, which releases nitric oxide to relax vascular smooth muscle. These substances are essential for maintaining coronary blood flow in response to increased myocardial oxygen demand.

**Why Each Wrong Option is Incorrect**
**Option A:** Adenosine is an obligate coronary vasodilator, as it causes significant dilation of coronary arteries by activating A2 receptors.
**Option B:** Nitroglycerin is also an obligate coronary vasodilator, as it releases nitric oxide to relax vascular smooth muscle and increase coronary blood flow.
**Option C:** (Insert correct option here)

**Clinical Pearl / High-Yield Fact**
Remember that obligate coronary vasodilators are essential for maintaining coronary blood flow in response to increased myocardial oxygen demand. This is particularly important in conditions such as coronary artery disease, where vasodilation can help to maintain blood flow to the ischemic myocardium.

**Correct Answer:** C.