Final answer:
The nitrogenous waste product produced from protein breakdown in the liver and excreted in urine as a small, water-soluble molecule is urea.
Step-by-step explanation:
The nitrogenous waste product that is a small, water-soluble molecule produced from protein breakdown in the liver is urea. This occurs as proteins are catabolized into amino acids, which are then deaminated, leading to the formation of ammonia, a highly toxic substance. To protect the organism, the liver rapidly converts most of the ammonia into less toxic urea through the urea cycle. The urea is then excreted from the body via urine.
This cycle is crucial because it helps to maintain a non-toxic environment in the body while facilitating the excretion of excess nitrogen.