**Core Concept**
Opioid-induced constipation is a common side effect of opioid therapy, caused by the activation of **mu receptors** in the gut, which decreases motility and increases water absorption. Treatment involves targeting these receptors or enhancing gut motility.

**Why the Correct Answer is Right**
The correct answer is likely a medication that targets the **mu receptors** or enhances gut motility, such as a peripherally acting mu-opioid receptor antagonist (**PAMORA**). This class of drugs can help mitigate constipation without affecting analgesia.

**Why Each Wrong Option is Incorrect**
**Option A:** May not be specific to opioid-induced constipation.
**Option B:** Could be a laxative, which may not address the underlying cause.
**Option C:** Might be an opioid itself, exacerbating constipation.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **PAMORAs** like methylnaltrexone are specifically used for opioid-induced constipation, as they do not cross the blood-brain barrier and thus do not interfere with opioid analgesia.

**Correct Answer:** D. Methylnaltrexone.