Which of the following substances is used to evaluate kidney filtration and function, is excreted, and the in reabsorbed?

• A. Urea
• B. Creatinine
• C. Glucose
• D. Sodium

Answer: (A)

Urea is useful for evaluating kidney filtration and function because its fate in the nephron combines both filtration and tubular handling. It is produced in the liver and filtered by the glomerulus, but a significant portion is reabsorbed as it passes through the tubules, especially when urine flow is low. This reabsorption means that the amount of urea removed from the blood depends not only on how well the glomerulus filters, but also on how much urea is reabsorbed in the tubules, which in turn is influenced by hydration and tubular function. Clinically, measuring urea in the blood (the BUN) provides a snapshot of renal function by reflecting these dynamics, and it is excreted in urine as part of the waste eliminated from the body.

In practice, this is often interpreted alongside creatinine. Creatinine is almost completely filtered with minimal reabsorption, making it a more direct indicator of glomerular filtration rate on its own. Glucose is normally fully reabsorbed in the tubules unless blood glucose is so high that its transport mechanisms are overwhelmed, so it isn’t used to gauge filtration. Sodium filtration and reabsorption are highly regulated and vary with diet and hormones, so sodium alone doesn’t provide a straightforward measure of overall kidney filtration.