The correct option is D Liver Urea is the major end product of nitrogen metabolism in humans and mammals. Ammonia, the product of oxidative deamination reactions, is toxic in even small amounts and must be removed from the body. The urea cycle or the ornithine cycle describes the conversion reactions of ammonia into urea. Since these reactions occur in the liver, the urea is then transported to the kidneys, where it is excreted. The overall urea formation reaction is: 2 Ammonia + carbon dioxide + 3ATP ---> urea + water + 3 ADP.