Which of the following is not a vasodilator?

Fentanyl is classified as an opioid analgesic, primarily used for pain management, rather than a vasodilator. Its primary action is to bind to opioid receptors in the brain, providing analgesia and anesthesia rather than affecting blood vessel dilation.

In contrast, hydralazine, minoxidil, and nitroprusside are well-known vasodilators used to manage hypertension and other cardiovascular conditions. Hydralazine works by directly relaxing vascular smooth muscle, while minoxidil induces vasodilation through its metabolite, which opens potassium channels. Nitroprusside acts as a potent vasodilator by releasing nitric oxide, leading to rapid reduction in blood pressure. Each of these agents specifically targets the smooth muscle of blood vessels, resulting in vasodilation.