A patient on long-term morphine for cancer pain develops severe constipation unresponsive to stimulant laxatives. Methylnaltrexone is prescribed. Its mechanism is:

• A Chloride channel activator in intestinal epithelium
• B Centrally acting mu-opioid receptor antagonist that reverses analgesia
• C Peripherally acting mu-opioid receptor antagonist that does not cross the blood-brain barrier ✓
• D Guanylate cyclase-C receptor agonist increasing intestinal fluid secretion

Explanation

Methylnaltrexone is a peripherally acting mu-opioid receptor antagonist (PAMORA) that does not cross the blood-brain barrier, so it reverses opioid-induced constipation without precipitating withdrawal or reversing analgesia. Lubiprostone activates chloride channels. Linaclotide and plecanatide act on guanylate cyclase-A.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.