What is the mechanism of action of hydralazine in obstetric hypertension?

• A. Beta Blocker leading to decreased heart rate
• B. ACE inhibitor causing arterial dilation
• C. Smooth muscle dilator leading to arterial dilation
• D. Calcium channel blocker reducing afterload

Answer: (C)

Hydralazine lowers blood pressure in obstetric hypertension by directly relaxing vascular smooth muscle in arterioles, causing arterial dilation. This direct arteriolar vasodilation reduces systemic vascular resistance and afterload, leading to a drop in blood pressure with relatively little venous dilation. In pregnancy, this rapid arterial dilation can cause reflex tachycardia and fluid retention, so monitoring and sometimes combining with other agents is common. It is not a beta blocker, ACE inhibitor, or calcium channel blocker; those mechanisms involve different targets, whereas hydralazine acts specifically on smooth muscle to dilate arteries.