What is excreted as a byproduct of protein metabolism in the kidneys?

The byproduct of protein metabolism that is excreted by the kidneys is uric acid. When proteins are metabolized, they are broken down into amino acids, which can then undergo deamination, a process in which the amino group is removed. This leads to the formation of ammonia, which is converted into urea and uric acid, especially when dealing with purines found in certain amino acids. Uric acid is generated from the breakdown of purines and is one of the waste products that the kidneys filter out of the blood to be excreted in urine.

In contrast, glucose, cholesterol, and fatty acids do not originate from protein metabolism. Glucose primarily comes from carbohydrates, cholesterol is a type of lipid synthesized in the liver and is not a direct product of protein metabolism, and fatty acids are components of lipids that do not arise as byproducts of protein catabolism. Therefore, uric acid stands out as the correct answer due to its direct relationship with protein breakdown and its subsequent excretion by the kidneys.