A patient in a hypertensive crisis is prescribed hydralazine. What is the primary action of hydralazine in treating hypertensive crisis?

A. Inhibits the renin-angiotensin-aldosterone system: Hydralazine does not act on the RAAS pathway and does not block angiotensin-converting enzyme or angiotensin receptors. Medications such as ACE inhibitors or ARBs are responsible for this mechanism. Hydralazine works peripherally on vascular smooth muscle. B. Promotes sodium and water retention: Sodium and water retention can occur as a compensatory adverse effect rather than a therapeutic action of hydralazine. This response is related to reflex activation of the sympathetic nervous system and RAAS. It does not explain the drug’s blood pressure–lowering effect. C. Increases heart rate to improve cardiac output: Reflex tachycardia may occur secondary to vasodilation, but it is not the intended or primary mechanism of action. Increasing heart rate can increase myocardial oxygen demand and is often an undesirable effect. This action does not directly reduce blood pressure. D. Directly causes arterial vasodilation to decrease blood pressure: Hydralazine relaxes arterial smooth muscle, leading to decreased systemic vascular resistance. This arterial vasodilation lowers afterload and rapidly reduces blood pressure in hypertensive crises. The effect is direct and does not rely on hormonal pathways.