Lactulose reduces ammonia by which mechanism?

• A. Converts ammonia to ammonium in the colon to be excreted
• B. Increases hepatic urea production
• C. Inhibits renal excretion of ammonia
• D. Lowers brain ammonia by crossing the blood-brain barrier

Answer: (A)

Lactulose lowers ammonia by changing the colon environment so ammonia becomes a form that can’t be absorbed. It’s not absorbed in the gut; bacteria ferment it to acids that acidify the colon. This acidic setting converts ammonia (NH3) to ammonium (NH4+), which is trapped in the lumen and excreted in stool. With less ammonia entering the bloodstream, blood ammonia levels fall and hepatic encephalopathy improves. The other options don’t fit: lactulose doesn’t directly boost hepatic urea production, it doesn’t inhibit renal excretion (it actually promotes elimination via stool), and while brain ammonia may decrease as a result, the mechanism isn’t about the drug crossing the blood-brain barrier.