Which substance is a waste product resulting from protein metabolism?

Urea is a key waste product that results from the metabolism of proteins in the body. During protein catabolism, amino acids are deaminated, which leads to the production of ammonia, a toxic byproduct. The liver then converts this ammonia into urea through the urea cycle, making it less toxic and more soluble for excretion. Urea is primarily eliminated from the body through the urine via the kidneys.

While ammonia itself is indeed a byproduct of protein metabolism, it is not the main waste product the body utilizes for excretion due to its toxicity. Creatinine is a metabolic waste product derived from the normal breakdown of muscle tissue, but it is not directly a result of protein metabolism itself. Glucose is a vital energy source and not a waste product; it is involved in carbohydrate metabolism rather than protein metabolism.

Thus, urea stands out as the primary waste product arising specifically from protein metabolism, making it the correct answer.