By which process is urea formed from ammonia in the liver?

Urea formation from ammonia in the liver is primarily accomplished through a process known as ureagenesis, which is also commonly referred to as the urea cycle. This metabolic pathway involves a series of enzymatic reactions that convert toxic ammonia, which is a byproduct of amino acid metabolism, into urea, a less toxic compound that can be safely excreted by the kidneys.

During ureagenesis, ammonia is combined with carbon dioxide in a series of steps to ultimately produce urea. This cycle is essential for the detoxification of ammonia, which can accumulate to harmful levels in the body if not converted to urea. The significance of this process cannot be overstated, as it plays a critical role in nitrogen metabolism and waste elimination in mammals.

Deamination and transamination are terms that relate to the processes involving the removal of amino groups from amino acids, but they do not specifically lead to the production of urea. Glutamine synthesis, on the other hand, involves the conversion of ammonia into glutamine, which is a different metabolic pathway primarily focused on detoxifying ammonia for transport and storage. Thus, ureagenesis is the correct choice that accurately describes the process by which ammonia is converted into urea in the liver.