A 58-year-old man with cirrhosis is treated with lactulose for hepatic encephalopathy. The therapeutic goal is titrated to produce two to three soft stools daily because the drug lowers blood ammonia by:
- A Binding ammonia directly in the systemic circulation as an inert complex
- B Inhibiting intestinal glutaminase activity
- C Acidifying colonic contents, converting NH3 to NH4+ which cannot be absorbed ✓
- D Enhancing hepatic urea cycle enzyme expression
Correct answer: C. Acidifying colonic contents, converting NH3 to NH4+ which cannot be absorbed
Explanation
Lactulose is a non-absorbable disaccharide metabolised by colonic bacteria to lactic and acetic acids. The resulting acidic luminal pH protonates NH3 to NH4+, which is trapped in the lumen and excreted in stool rather than absorbed into portal blood. Rifaximin complements this by reducing ammonia-producing bacteria. Lactulose does not bind circulating ammonia, inhibit glutaminase, or act on the liver itself.
Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.
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