**Core Concept**
Post-operative shivering is a common complication that can occur after anesthesia, leading to increased oxygen consumption, cardiac workload, and discomfort for the patient. The underlying mechanism involves the body's thermoregulatory response to the cold stress caused by anesthesia.

**Why the Correct Answer is Right**
The correct answer involves the use of a medication that targets the sympathetic nervous system to alleviate shivering. Dexmedetomidine, a selective α2-adrenergic receptor agonist, is used to treat post-operative shivering by inhibiting the release of norepinephrine from sympathetic nerve terminals. This results in a decrease in the body's thermoregulatory response, thereby reducing shivering.

**Why Each Wrong Option is Incorrect**
**Option A:** Haloperidol is an antipsychotic medication that can cause extrapyramidal side effects, but it is not used to treat post-operative shivering. It is not a suitable choice for this indication.
**Option B:** Midazolam is a benzodiazepine used for sedation and anxiolysis, but it is not specifically used to treat post-operative shivering. Its mechanism of action does not directly target the sympathetic nervous system.
**Option C:** Fentanyl is an opioid analgesic, but it is not typically used to treat post-operative shivering. While it can provide analgesia, its mechanism of action does not directly address the thermoregulatory response.

**Clinical Pearl / High-Yield Fact**
When managing post-operative shivering, it's essential to consider the underlying cause and choose a medication that targets the sympathetic nervous system. Dexmedetomidine is a useful option in this scenario, but other medications like meperidine or buspirone may also be considered.

**Correct Answer: D. Dexmedetomidine**