Which metabolite is produced from muscle breakdown and excreted by the kidneys?

The correct metabolite produced from muscle breakdown and excreted by the kidneys is creatinine. Creatinine is a byproduct of creatine phosphate metabolism in muscle tissue. It is formed at a relatively constant rate depending on muscle mass and is released into the bloodstream, where it is then filtered and excreted by the kidneys.

The significance of creatinine lies in its use as a clinical marker for kidney function. Elevated levels of creatinine in the blood may indicate impaired renal function, as the kidneys' ability to filter and excrete it would be compromised. Therefore, measuring creatinine levels can provide valuable information regarding how well the kidneys are working.

Other metabolites listed play different roles: urea is a byproduct of protein metabolism rather than muscle breakdown, uric acid is related to the breakdown of purines and is associated with conditions like gout, and ammonia is primarily a product of protein catabolism that is converted to urea by the liver for excretion, but it is not directly linked to muscle breakdown. This context further underscores the distinct role of creatinine as a specific marker related to muscle metabolism and renal function.