**Core Concept**
Clonidine is an **antihypertensive** agent that works by stimulating **alpha-2 adrenergic receptors** in the brain, leading to a decrease in sympathetic outflow and subsequently lowering blood pressure. However, its effects can vary based on the method of administration.

**Why the Correct Answer is Right**
When clonidine is given as a fast intravenous injection, it can initially stimulate **alpha-2 adrenergic receptors** on the vascular smooth muscle, causing vasoconstriction and an increase in blood pressure. This initial response is due to a direct effect on these receptors before the central effects of decreased sympathetic outflow can occur.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is not provided, so we cannot assess its accuracy.
**Option B:** Similarly, without the specific details of option B, we cannot determine why it is incorrect.
**Option C:** Assuming this option does not correctly identify the mechanism by which clonidine increases blood pressure with rapid IV administration, it would be incorrect.
**Option D:** Like options A and B, without specifics, we cannot evaluate the correctness of this choice.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that the method of administration of a drug can significantly affect its pharmacological response. In the case of clonidine, understanding its dual effects based on administration rate is key to avoiding unexpected hypertensive responses.

**Correct Answer:** D. Initial peripheral vasoconstriction due to alpha-2 adrenergic receptor stimulation on vascular smooth muscle.