Final answer:
Urea is the crystalline compound produced from protein metabolism in mammals, which leaves the body in urine as the primary nitrogenous waste.
Step-by-step explanation:
The crystalline compound that is a product of protein metabolism in mammals is known as urea. Proteins are first broken down into amino acids, which can then enter various metabolic pathways.
When there is an excess of amino acids or during starvation, the amino acids may be directed towards glucose catabolism, where the amino group is removed and converted to ammonia. In mammals, the liver processes this ammonia, along with carbon dioxide, to produce urea. Urea then leaves the body through urine, serving as the primary nitrogenous waste.