Which parenteral vasodilator is primarily associated with reflex tachycardia and fluid retention as adverse effects?

• A. Nitroprusside
• B. Labetalol
• C. Minoxidil
• D. Hydralazine

Answer: (B)

When a parenteral vasodilator lowers systemic vascular resistance quickly, the body activates the baroreceptor reflex and the renin–angiotensin system. This compensatory response increases sympathetic outflow, causing reflex tachycardia, and promotes sodium and water retention, leading to fluid retention. Direct arteriolar dilators are the drugs most commonly associated with these effects. Among the options, hydralazine is the classic parenteral vasodilator known for causing both tachycardia and edema due to this reflex and RAAS activation. Labetalol, with alpha and beta blocking, tends to blunt reflex tachycardia and does not typically promote fluid retention; minoxidil can also cause strong reflex tachycardia and fluid retention but is less commonly used parenterally, and nitroprusside can trigger tachycardia but fluid retention is not its hallmark. So the drug that best fits the described adverse effects is hydralazine.