Which antidiarrheal agent acts by inhibiting prostaglandin synthesis and is contraindicated in infectious diarrhea?

Explanation

## Bismuth Subsalicylate: Mechanism and Contraindications ### Mechanism of Action Bismuth subsalicylate works through multiple mechanisms: 1. **Prostaglandin synthesis inhibition** — the salicylate component reduces PG production, which decreases intestinal secretion and motility 2. **Antimicrobial effect** — bismuth has direct bactericidal activity against some organisms 3. **Anti-inflammatory effect** — reduces mucosal inflammation ### Contraindication in Infectious Diarrhea **Key Point:** Bismuth subsalicylate is contraindicated in infectious diarrhea (especially bacterial and parasitic) because: - Inhibiting prostaglandins reduces protective mucosal secretions - Decreased motility may trap pathogens and toxins in the bowel - Risk of toxic megacolon and severe complications increases - Particularly dangerous in *Shigella*, *Salmonella*, and *Clostridium difficile* infections ### Comparison of Antidiarrheal Agents | Agent | Mechanism | Safe in Infectious Diarrhea? | Notes | | --- | --- | --- | --- | | **Bismuth subsalicylate** | PG inhibition + antimicrobial | **NO** — increases complication risk | Salicylate component is key | | Loperamide | μ-opioid agonist (increases sphincter tone) | Relatively safer but still caution | Less risk than bismuth; avoid in severe infection | | Diphenoxylate | μ-opioid agonist + atropine | Relatively safer but still caution | Opioid component; avoid in severe infection | | Racecadotril | Enkephalinase inhibitor | Safer — no motility reduction | Preserves normal gut motility | **Warning:** Do NOT use bismuth subsalicylate in suspected bacterial or parasitic diarrhea. The combination of PG inhibition and antimotility effects increases the risk of invasive complications and toxic megacolon. **Clinical Pearl:** Bismuth subsalicylate is useful for traveler's diarrhea (prophylaxis and mild cases) and nonspecific diarrhea, but must be avoided once bacterial or parasitic infection is suspected. **High-Yield:** The key distinguishing feature of bismuth subsalicylate is its **prostaglandin-inhibiting** property via the salicylate moiety — this is why it is contraindicated in infectious diarrhea.