Which of the following is NOT a direct vasodilator?

Ramipril is not classified as a direct vasodilator. Instead, it is an angiotensin-converting enzyme (ACE) inhibitor. ACE inhibitors work by blocking the conversion of angiotensin I to angiotensin II, a potent vasoconstrictor. By inhibiting this conversion, ramipril leads to a decrease in angiotensin II levels, which results in vasodilation and a reduction in blood pressure. This indirect vasodilatory effect distinguishes it from direct vasodilators, which directly relax the smooth muscle in blood vessels without the intermediary effects of altering hormone levels.

In contrast, hydralazine, nitroprusside, and milrinone all serve as direct vasodilators. Hydralazine acts directly on the arterial smooth muscle, causing relaxation and vasodilation. Nitroprusside also directly relaxes vascular smooth muscle, leading to rapid vasodilation. Milrinone, while primarily a phosphodiesterase inhibitor with inotropic effects, also exhibits direct vasodilatory properties by increasing intracellular cAMP, which contributes to its vasodilation effect. Thus, ramipril stands apart due to its mechanism of action as an ACE inhibitor and not as a direct vasodilator.