Carbamoyl phosphate synthetase is the rate-limiting step of the urea cycle. It is the first step of the urea cycle and occurs exclusive in the mitochondria of hepatic and renal cells.

2 molecules of ammonia and 1 molecule of carbon dioxide are converted into 1 molecule of urea in every turn of the urea cycle. In addition, each cycle regenerates 1 molecule of ornithine for use in the next turn.

Removal of the amino group is a primary step in amino acid catabolism. Humans (and some other animals) utilize the urea cycle to convert the unnecessary amino groups from amino acids into urea. The carbon skeletons will be fed into the citric acid cycle, and none of the other answers directly relate to human nitrogen metabolism. The Krebs cycle (also known as the citric acid cycle) is an intermediate between glycolysis and oxidative phosphorylation, and involves energy production. Gluconeogenesis is the process by which glucose is synthesized from non-carbohydrate organic substrates. The purine nucleotide cycle involves conversion of nucleotides into Krebs cycle intermediates, and makes ammonia as a byproduct, which is then shunted into the urea cycle to be converted into urea and excreted in the urine.

Aminotransferases (transaminases) catalyze the conversion of an amino acid to an -keto acid. To do so, they utilize a pyridoxyl phosphate coenzyme, which mediates the transfer of the amino group, leaving a ketone functional group next to the carboxyl group. This -keto acid is the product. Since the amino group in an amino acid is attached to the alpha carbon, it will not be a -keto acid. Some of the other answer choices give intermediates of the reaction mechanism, not the product.

Due to its amino group's location within the pyrrole ring derivative, proline is unable to be deaminated via the aminotransferase step of protein catabolism. Proline oxidase is the enzyme that catalyzes the first reaction in the catabolism of proline. None of the other amino acid answer choices have this limitation.

When proteins and amino acids are broken down in the body, ammonium is created as a byproduct. Ammonium is dangerous when it remains free in the human body, so something must be done to get rid of it. The major route of removal of ammonium by the body is via urea synthesis in the liver. Urea can then be excreted in urine.

Both catabolic and anabolic reactions are metabolic reactions. The difference is that catabolism is when complex molecules break down into simpler molecules, and anabolism is when simpler molecules are combined to form complex molecules. ATP (adenosine triphosphate) is used to store and transport energy in cells. There is a major difference in how ATP is used in catabolic and anabolic reactions. Anabolic reactions require energy input, and result in a net consumption of ATP. Catabolic reactions produce energy and results in a net synthesis of ATP. Therefore, catabolic pathways do not have a net consumption of ATP.

The urea cycle is vital to the excretion of ammonia, a harmful byproduct of amino acid breakdown. Via a series of enzymatic changes, ammonia is converted to urea, which can be excreted into the urine. 

The urea cycle occurs primarily in the liver, and to a lesser extent in renal cells. There is no urea conversion performed by the small intestine. 

Arginine is the amino acid in the cycle that is converted to urea and ornithine via the enzyme arginase, and is one of the products of the lysis of argininosuccinate. Glutamic acid plays a different role in the cycle; it loses its amino group to the synthesis of carbamoyl phosphate, a precursor of citrulline. Asparagine is not present in the urea cycle, but aspartic acid is. It condenses with citrulline, through the action of the enzyme argininosuccinate synthase and ATP, to produce argininosuccinate.