**Core Concept**
Reflex tachycardia is a physiological response to a decrease in blood pressure, mediated by the **baroreceptor reflex**. This reflex aims to maintain adequate blood flow to vital organs. **Vasodilators** can trigger this response by decreasing peripheral resistance.

**Why the Correct Answer is Right**
The correct answer is related to the administration of a **vasodilator**, which decreases peripheral vascular resistance, leading to a drop in blood pressure. The body compensates for this decrease by increasing heart rate, thereby maintaining cardiac output. This is achieved through the **sympathetic nervous system**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not directly cause vasodilation.
**Option B:** Incorrect as it is not typically associated with reflex tachycardia.
**Option C:** Incorrect because it is not the most direct cause of reflex tachycardia.

**Clinical Pearl / High-Yield Fact**
Reflex tachycardia is a common response to **antihypertensive medications**, especially those that act as vasodilators. Recognizing this relationship is crucial for managing patients' blood pressure and heart rate.

**Correct Answer:** D. a vasodilator