GIT pharmacology: antidiarrheals and laxatives MCQs With Answer

Introduction: This set of MCQs on GIT pharmacology focuses on antidiarrheals and laxatives tailored for M.Pharm students preparing for Advanced Pharmacology-II. The questions probe mechanisms of action, pharmacokinetics, clinical indications, adverse effects, drug interactions and recent targeted agents used in constipation and diarrhea management. Emphasis is placed on drug classes such as opioid receptor agonists/antagonists, adsorbents, antisecretory agents, bulk-forming, osmotic and stimulant laxatives, and newer drugs like lubiprostone, linaclotide and PAMORAs. These practice questions are designed to deepen conceptual understanding, encourage critical thinking about therapeutic choices, and reinforce knowledge required for exam-style problem solving and evidence-based clinical applications.

Q1. Which of the following correctly describes the primary mechanism by which loperamide reduces diarrhea?

• Stimulates enteric serotonin receptors to enhance intestinal motility
• Peripheral μ-opioid receptor agonist that slows intestinal motility and increases transit time
• Inhibits bacterial toxins by binding to endotoxins in the gut lumen
• Acts as an osmotic agent drawing water into the intestinal lumen

Correct Answer: Peripheral μ-opioid receptor agonist that slows intestinal motility and increases transit time

Q2. Why does loperamide have low central opioid effects at therapeutic doses?

• It is rapidly metabolized by hepatic CYP2D6 and eliminated before reaching the brain
• It is a P-glycoprotein (P-gp) substrate that is actively expelled at the blood–brain barrier
• It binds tightly to intestinal mucosa preventing systemic absorption
• It selectively binds κ-opioid receptors which are not present in CNS

Correct Answer: It is a P-glycoprotein (P-gp) substrate that is actively expelled at the blood–brain barrier

Q3. Which antidiarrheal agent acts by inhibiting enkephalinase and thereby enhances endogenous enkephalins to reduce secretion?

• Bismuth subsalicylate
• Racecadotril (thiorphan prodrug)
• Diphenoxylate
• Kaolin-pectin

Correct Answer: Racecadotril (thiorphan prodrug)

Q4. Bismuth subsalicylate exerts antidiarrheal effects primarily via which mechanisms?

• Central μ-opioid receptor agonism and decreased motility
• Antisecretory action, antimicrobial effects and mucosal protective coating
• Inhibition of Na+/K+ ATPase in intestinal epithelial cells
• Stimulation of chloride channels to increase water secretion

Correct Answer: Antisecretory action, antimicrobial effects and mucosal protective coating

Q5. Which class of laxatives requires adequate fluid intake to be effective and can cause intestinal obstruction if fluid is insufficient?

• Bulk-forming laxatives (e.g., psyllium)
• Stimulant laxatives (e.g., senna)
• Lubricant laxatives (e.g., mineral oil)
• Osmotic laxatives (e.g., polyethylene glycol)

Correct Answer: Bulk-forming laxatives (e.g., psyllium)

Q6. Which laxative acts primarily by drawing water into the colon via osmotic activity and is safe for chronic use with minimal systemic absorption?

• Magnesium hydroxide
• Bisacodyl
• Lubiprostone
• Polyethylene glycol (PEG)

Correct Answer: Polyethylene glycol (PEG)

Q7. Lubiprostone’s mechanism of action is best described as:

• μ-opioid receptor antagonism in the gut
• Activation of ClC-2 chloride channels leading to increased intestinal fluid secretion
• Inhibition of bacterial enterotoxins to reduce secretory diarrhea
• Guanylate cyclase-C agonism increasing intracellular cGMP

Correct Answer: Activation of ClC-2 chloride channels leading to increased intestinal fluid secretion

Q8. Linaclotide improves constipation primarily by which mechanism?

• Stimulating enteric motility through 5-HT4 receptor agonism
• Activating guanylate cyclase-C on enterocytes to increase cGMP and chloride secretion
• Acting as a mu-opioid receptor antagonist to restore normal motility
• Inhibiting colonic sodium channels leading to water retention in stool

Correct Answer: Activating guanylate cyclase-C on enterocytes to increase cGMP and chloride secretion

Q9. Which of the following is a peripherally acting μ-opioid receptor antagonist (PAMORA) used to treat opioid-induced constipation without reversing central analgesia?

• Methylnaltrexone
• Loperamide
• Docusate sodium
• Psyllium husk

Correct Answer: Methylnaltrexone

Q10. Which stimulant laxative exerts its effect by stimulating enteric nerves and altering fluid secretion secondary to mucosal irritation?

• Senna (sennosides)
• Psyllium
• Lactulose
• Docusate sodium

Correct Answer: Senna (sennosides)

Q11. Which of the following agents is an antisecretory drug that inhibits intestinal prostaglandin and chloride secretion and is commonly used for traveler’s diarrhea?

• Bismuth subsalicylate
• Octreotide
• Magnesium citrate
• Ondansetron

Correct Answer: Bismuth subsalicylate

Q12. Octreotide is particularly useful in diarrhea due to which of the following causes?

• Osmotic diarrhea from laxatives
• Secretory diarrhea due to VIPoma, carcinoid syndrome, or chemotherapy-induced secretory diarrhea
• Acute infectious diarrhea due to E. coli that responds to adsorbents
• Constipation-predominant irritable bowel syndrome

Correct Answer: Secretory diarrhea due to VIPoma, carcinoid syndrome, or chemotherapy-induced secretory diarrhea

Q13. Which laxative is a stool surfactant that lowers surface tension to allow water and lipids to penetrate fecal mass?

• Docusate sodium (stool softener)
• Bisacodyl (stimulant)
• Magnesium sulfate (osmotic)
• Psyllium (bulk-forming)

Correct Answer: Docusate sodium (stool softener)

Q14. Which of the following is a common and clinically important adverse effect of stimulant laxatives when used chronically?

• Hypokalemia and possible cathartic colon with impaired colonic motility
• Systemic opioid-like sedation
• Permanent enamel erosion
• Increased risk of small intestinal bacterial overgrowth (SIBO)

Correct Answer: Hypokalemia and possible cathartic colon with impaired colonic motility

Q15. Which antidiarrheal is contraindicated in children with viral illnesses due to risk of Reye-like syndrome from the salicylate component?

• Loperamide
• Bismuth subsalicylate
• Diphenoxylate-atropine
• Racecadotril

Correct Answer: Bismuth subsalicylate

Q16. Which osmotic laxative is metabolized by colonic bacteria to short-chain fatty acids, acidifying the lumen and increasing osmotic pressure to stimulate motility?

• Lactulose
• Polyethylene glycol
• Magnesium hydroxide
• Sodium picosulfate

Correct Answer: Lactulose

Q17. A serious cardiac adverse effect associated with high-dose loperamide misuse or interaction with CYP3A4/P-gp inhibitors is:

• Atrial fibrillation
• QT interval prolongation and risk of torsades de pointes
• Complete heart block due to vagal stimulation
• Myocardial ischemia from coronary vasospasm

Correct Answer: QT interval prolongation and risk of torsades de pointes

Q18. Which antidiarrheal or supportive agent binds bacterial toxins and provides a protective coating, but has limited efficacy for secretory diarrhea?

• Kaolin-pectin
• Octreotide
• Racecadotril
• Loperamide

Correct Answer: Kaolin-pectin

Q19. For a patient with opioid-induced constipation unresponsive to laxatives, which therapeutic approach is most appropriate to reverse peripheral opioid effects while preserving analgesia?

• High-dose oral naloxone to antagonize central and peripheral opioid receptors
• Switching opioid to a different opioid with less constipating effect only
• Administration of a peripherally acting μ-opioid receptor antagonist (e.g., methylnaltrexone or naloxegol)
• Giving bulk-forming fiber supplements alone without additional drugs

Correct Answer: Administration of a peripherally acting μ-opioid receptor antagonist (e.g., methylnaltrexone or naloxegol)

Q20. Which laxative is contraindicated in patients with suspected bowel obstruction due to risk of perforation from increased intraluminal pressure?

• Psyllium (bulk-forming agent)
• Docusate sodium
• Mineral oil
• Lactulose

Correct Answer: Psyllium (bulk-forming agent)

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