**Question:** Which of the following is not a vasodilator?

A. Nitroglycerin
B. Norepinephrine
C. Epinephrine
D. Prostaglandin

**Core Concept:** Vasodilators are substances that cause the dilation (widening) of blood vessels, leading to a decrease in blood pressure and an increase in blood flow. This can be achieved through various mechanisms, including direct relaxation of smooth muscle, inhibition of the sympathetic nervous system, or stimulation of nitric oxide production.

**Why the Correct Answer is Right:** D. Prostaglandin is a hormone that plays a crucial role in inflammation, pain, and fever regulation. While prostaglandins can constrict blood vessels in some cases (e.g., prostaglandin E2 in uterine contractions), they primarily function as vasoconstrictors. Therefore, prostaglandin is not a typical vasodilator.

**Why Each Wrong Option is Incorrect:**

B. Norepinephrine is a strong vasoconstrictor that increases blood pressure and heart rate. It acts on alpha and beta-adrenergic receptors in the vascular smooth muscle to contract the blood vessels.

C. Epinephrine (adrenaline) is another vasoconstrictor, acting similarly to norepinephrine in increasing blood pressure and heart rate. It also constricts blood vessels through interaction with alpha and beta receptors.

A. Nitroglycerin is a vasodilator that lowers blood pressure by causing relaxation of vascular smooth muscle, allowing blood vessels to dilate (widen). Nitroglycerin is commonly used in the treatment of angina and hypertension.

D. Prostaglandin, as mentioned before, primarily functions as a vasoconstrictor rather than a vasodilator.

**Clinical Pearl:** Understanding the classification of vasodilators is essential for understanding cardiovascular physiology and pharmacotherapy. Vasodilators can be broadly categorized as nitrates, calcium channel blockers, or endothelium-derived substances (EDS). Nitroglycerin belongs to the nitrate group, while prostaglandins are part of the EDS.