Which metabolic waste is primarily eliminated by the kidneys?

• A. Bile pigments
• B. Urea
• C. Bilirubin
• D. Glucose

Answer: (B)

The kidneys are the main route for excreting waste products from protein metabolism. The principal nitrogenous waste produced from amino acid breakdown is urea. In the liver, ammonia is converted to urea, which is water-soluble and readily filtered by the kidneys into urine. This makes urea the primary metabolic waste eliminated by the kidneys.

Bile pigments, like bilirubin, come from the breakdown of heme and are processed by the liver for elimination in bile and feces, not primarily through urine. Glucose is a useful energy source that is normally filtered and then reabsorbed by the kidney tubules; it is not a waste product unless levels are abnormally high, in which case its presence in urine indicates a problem.