**Core Concept**
Postoperative shivering is a common complication that can occur after general anesthesia, leading to increased oxygen consumption, cardiac workload, and potentially even postoperative complications. The treatment of postoperative shivering involves pharmacological interventions that target the thermoregulatory pathways in the brain.

**Why the Correct Answer is Right**
The correct answer, **Dexmedetomidine**, is a selective alpha-2 adrenergic agonist that has been shown to effectively reduce shivering in postoperative patients. Dexmedetomidine acts by stimulating alpha-2 adrenergic receptors in the brain, which leads to a decrease in the release of norepinephrine and other neurotransmitters involved in the shivering response. This results in a reduction in body temperature and a decrease in the incidence of shivering. Other agents, such as meperidine and clonidine, have also been used to treat postoperative shivering, but dexmedetomidine is a more effective and safer option.

**Why Each Wrong Option is Incorrect**
**Option A:** **Meperidine** is an opioid analgesic that can be used to treat pain, but it is not the most effective agent for treating postoperative shivering. Meperidine can actually increase the risk of shivering in some patients.
**Option B:** **Clonidine** is an alpha-2 adrenergic agonist, but it is not as effective as dexmedetomidine in reducing shivering. Clonidine has a higher risk of sedation and other side effects compared to dexmedetomidine.
**Option C:** **Nitroglycerin** is a vasodilator that is used to treat angina and hypertension, but it has no role in the treatment of postoperative shivering.

**Clinical Pearl / High-Yield Fact**
Dexmedetomidine is a useful agent for treating postoperative shivering due to its ability to reduce the release of norepinephrine and other neurotransmitters involved in the shivering response. However, it should be used with caution in patients with hypotension, bradycardia, or other cardiovascular instability.

**Correct Answer: D. Dexmedetomidine**