Identify the nitrogenous excretory waste based on the following features.

a. Excretion of this substance is a terrestrial adaptation.

b. Some amount of this substance may be retained in the kidney matrix to maintain a desired osmolarity.

c. This is the chief nitrogenous waste excreted by some terrestrial animals as well as some aquatic animals.

d. It is synthesised in the liver.

Terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes like urea (and uric acid) for conservation of water. Ammonia produced by metabolism is converted into urea in the liver of ureotelic animals and released into the blood which is filtered and excreted out by the kidneys. Some amount of urea may be retained in the kidney matrix to maintain a desired osmolarity. Mammals, many terrestrial amphibians and marine fishes are ureotelic.