**Core Concept**
Opioid-induced constipation is a common side effect of opioid medications, resulting from the inhibition of gastrointestinal motility and water secretion. This occurs due to the activation of mu-opioid receptors in the enteric nervous system, which reduces the release of acetylcholine, a neurotransmitter involved in gut contractions.

**Why the Correct Answer is Right**
The treatment of opioid-induced constipation involves the use of medications that counteract the effects of opioids on the enteric nervous system. One such medication is methylnaltrexone, a peripherally acting mu-opioid receptor antagonist. Methylnaltrexone selectively blocks mu-opioid receptors in the gastrointestinal tract, thereby increasing gut motility and water secretion, without affecting the central nervous system effects of opioids.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not specify a medication that targets the enteric nervous system to treat opioid-induced constipation.
* **Option B:** Incorrect, as this medication is not specifically used to treat opioid-induced constipation.
* **Option D:** This option is incorrect as it does not address the specific mechanism of opioid-induced constipation in the gastrointestinal tract.

**Clinical Pearl / High-Yield Fact**
Methylnaltrexone is a valuable treatment option for opioid-induced constipation, as it can be administered subcutaneously or orally, and has a rapid onset of action, making it an effective choice for managing this common side effect in patients on opioid therapy.

**Correct Answer: C. Methylnaltrexone**