**Core Concept**
Alpha-2 (a2) adrenergic agonists are a class of medications that activate a2 adrenergic receptors, which are G protein-coupled receptors that regulate various physiological processes. These receptors are involved in the inhibition of neurotransmitter release, particularly norepinephrine, and are found in both the central nervous system and the peripheral nervous system.

**Why the Correct Answer is Right**
a2 agonists, such as clonidine, dexmedetomidine, and tizanidine, cause a decrease in sympathetic tone, leading to a decrease in heart rate, blood pressure, and cardiac output. They also cause sedation, analgesia, and a decrease in anxiety. Additionally, a2 agonists can cause miosis (pupil constriction) due to the inhibition of norepinephrine release in the iris. The correct answer choice should be a side effect or action that is not associated with a2 agonist activity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a2 agonists can indeed cause hypotension, which is a decrease in blood pressure. This is due to the decrease in sympathetic tone and the subsequent decrease in vascular resistance.

**Option B:** This option is incorrect because a2 agonists can cause sedation and sedative effects are a well-documented consequence of a2 agonist activity.

**Option C:** This option is incorrect because a2 agonists can cause miosis, which is a decrease in pupil size due to the inhibition of norepinephrine release in the iris.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that a2 agonists can cause rebound hypertension if abruptly discontinued, particularly in patients with a history of hypertension. This is due to the abrupt withdrawal of the a2 agonist, leading to an increase in sympathetic tone and a subsequent increase in blood pressure.

**Correct Answer: D.**