**Core Concept**
Phenylephrine is a selective α1-adrenergic receptor agonist, causing vasoconstriction and an increase in blood pressure. However, its administration can also lead to reflex bradycardia due to the activation of baroreceptors in the aortic arch and carotid sinus, which send signals to the brainstem to slow down the heart rate.

**Why the Correct Answer is Right**
The correct answer is a drug that can block the vagal reflex, thereby preventing the decrease in heart rate caused by Phenylephrine. This can be achieved by administering a drug that blocks the action of acetylcholine at the muscarinic receptors in the heart, which are responsible for the bradycardic effect. **Atropine** is a non-selective muscarinic receptor antagonist that can effectively block the vagal reflex and prevent the decrease in heart rate caused by Phenylephrine.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not address the specific mechanism of action of Phenylephrine and its effect on heart rate.
* **Option B:** This option is incorrect because it is not a direct antagonist of the vagal reflex, and its mechanism of action is not relevant to the context of Phenylephrine administration.
* **Option D:** This option is incorrect because it is not a specific antagonist of the muscarinic receptors in the heart and does not address the mechanism of action of Phenylephrine.

**Clinical Pearl / High-Yield Fact**
When administering Phenylephrine, it is essential to consider the potential for reflex bradycardia, especially in patients with compromised cardiovascular function. Atropine can be used to prevent or treat this effect, but its administration should be judicious and based on clinical assessment of the patient's needs.

**Correct Answer:** A. Atropine