A patient with recurrent episodes of overt hepatic encephalopathy is prescribed regular lactulose titrated to two to three soft stools per day. By what principal mechanism does lactulose reduce blood ammonia?

• A Colonic acidification converts ammonia to non-absorbable ammonium ✓
• B Direct binding of ammonia by synthetic disaccharide residues
• C Inhibition of urease-producing colonic bacteria
• D Stimulation of renal ammonium excretion through osmotic diuresis

Explanation

Lactulose reaches the colon undigested, is fermented to organic acids, lowers stool pH, and shifts the equilibrium from diffusible ammonia toward ammonium ions that cannot cross the colonic epithelium; the cathartic effect also shortens transit and reduces ammonia absorption time. It does not chemically bind ammonia nor primarily work through antibacterial action, which is the domain of rifaximin used alongside it for recurrence prophylaxis.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.