Describe ammonia metabolism and excretion (70% of marks). Outline the 
pharmacology of lactulose (30% of marks).

[Click here to toggle visibility of the answers]

College Answer

It was expected candidates would identify sources of ammonia (colon from metabolism of 
proteins, kidney, small amounts from breakdown of red blood cells and metabolism in muscles). 
The liver converts all circulating ammonia to urea (the urea cycle) (2NH3+CO2 = urea +H2O). Urea then undergoes enterohepatic circulation (25%) or is excreted by kidneys (75%). 
Ammonia (NH3) is lipid soluble and diffuses into the interstitial cell and tubular fluid by non-ionic 
diffusion where is buffers H+ to become non diffusible NH4+. No candidate mentioned 
enterohepatic circulation and most answers had very little detail on the metabolism and 
excretion and lacked depth.
Lactulose is a non absorbable synthetic, non-digestible disaccharide. It is an osmotic laxative 
fermented by gut flora producing metabolites (such as acetate) which have osmotic and 
peristalsis-stimulating effects, and methane causing in flatulence.
Few could describe how lactulose decrease absorption of ammonia and a surprising number of 
people did not even state that lactulose was an osmotic laxative.