The correct answer is Methylnaltrexone.

Explanation:

Opioid-induced constipation (OIC) is a common side effect of opioid therapy due to the binding of opioids to peripheral μ-opioid receptors in the gastrointestinal tract, which reduces bowel motility and secretion. Treatment of OIC requires medications that specifically target these peripheral receptors without affecting central analgesia.

- Methylnaltrexone is a peripherally acting μ-opioid receptor antagonist (PAMORA). It selectively blocks opioid receptors in the gut, thereby reversing opioid-induced constipation without crossing the blood-brain barrier or interfering with opioid-mediated pain relief. This makes it specifically indicated for treating constipation caused by opioid use.

- Naloxegol is another PAMORA that also treats opioid-induced constipation and is structurally related to naloxone, but since the correct answer here is methylnaltrexone, it's important to note that both drugs share a similar role.

- Loperamide is an opioid agonist that acts on μ-opioid receptors in the gut to slow gastrointestinal motility and is used to treat diarrhea, not constipation.

- Bisacodyl is a stimulant laxative that induces bowel movements by stimulating enteric nerves but is not specifically indicated for opioid-induced constipation and does not address the underlying receptor mechanism.

In summary, methylnaltrexone's unique mechanism of selectively antagonizing peripheral opioid receptors without impacting analgesia makes it the preferred treatment for opioid-induced constipation.

Reference: Goodman & Gilman's The Pharmacological Basis of Therapeutics, 13th Edition, Chapter 23: Drugs Acting on the Gastrointestinal Tract and the Liver / Page 672-675