This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Other topics and Adverse effects, Subject name: Pharmacology
Which of the following laxatives lowers blood ammonia level in hepatic encephalopathy? 
Select and explain. 
A. Bisacodyl, B. Liquid paraffin, C. Lactulose, D. Magnesium sulfate
C: Lactulose
Answer: C: Lactulose 
Explanation: (Ref: KDT 6/e p655) Lactulose is degraded to lactic acid that conves NH3 to NH4+. As ionic molecules cannot cross biological membranes, it is not absorbed and is thus excreted.