An endogenous substance used to assess glomerular filtration rate is described as filtered by the glomeruli, not reabsorbed by the tubules, and only secreted when plasma levels are elevated. Which substance is this?

• A. Inulin
• B. Uric acid
• C. Creatinine
• D. Urea

Answer: (C)

Estimating GFR relies on substances whose renal handling mainly reflects filtration with minimal interference from reabsorption or secretion. Creatinine fits this pattern. It’s produced endogenously in muscle, is freely filtered by the glomeruli, and is not reabsorbed in the tubules. A small amount is secreted by the proximal tubule, and this secretion increases as plasma levels rise, which is why creatinine clearance is a close, practical proxy for GFR even though it slightly overestimates true GFR due to that secretion.

Inulin would be ideal for a precise GFR measurement because it’s filtered but neither reabsorbed nor secreted, yet it is not endogenous and must be infused. Urea is substantially reabsorbed in the tubules, so its clearance underestimates GFR and is not reliable for assessing filtration. Uric acid undergoes both filtration and variable reabsorption/secretion, making its clearance poorly correlated with GFR.