Hint: Organisms accumulate urea or some other substances such as ammonia, uric acid, carbon dioxide, water, etc. as a result of metabolic activities. Among these, ammonia, urea, and uric acid are the most toxic nitrogenous wastes excreted by the animals.
Urea is comparatively less toxic than ammonia. It is also known as carbamide. Urea is the principal nitrogenous waste excreted by many terrestrial amphibians, mammals, and marine fishes. At the end of chain reactions of the metabolic pathway, amino acids that make the proteins are broken down and converted into ammonia, carbon dioxide, water, and energy in the liver. As ammonia is very toxic thus, it is needed to be excreted out of the body. Urea is excreted in the form of urine from the body as it is highly soluble in water. Thus, we can say that urea is derived from amino acids.
Hence, the correct answer is option (B).
The animals that excrete urea are known as ureotelic animals. In ureotelic, the ammonia and carbon dioxide produced as a result of metabolic activities are first converted into urea in the liver, and then urea is released into the blood. The blood is then filtered and is finally excreted out of the kidney. This process of conversion of ammonia and carbon dioxide into urea is known as the urea cycle. Special enzymes and energy are required to complete the reactions of the urea cycle. As urea is less toxic, it can get accumulated in the tissues at higher concentrations without damaging the tissue. It can also be excreted in a more concentrated form.
Note: On average, a human being excretes approximately 25 grams of urea in a day. After excretion, bacteria convert urea again into ammonia. Any impairment that causes difficulty in the elimination of urea leads to a serious problem known as uremia.