Inulin clearance is used to measure GFR because inulin is which of the following?

• A. Secreted by the tubules
• B. Reabsorbed by the tubules
• C. Freely filtered and neither secreted nor reabsorbed
• D. Metabolized in the kidney

Answer: (C)

Inulin is used to measure GFR because it behaves as an ideal filtration marker: it is freely filtered at the glomerulus and then neither reabsorbed nor secreted by the renal tubules, and it is not metabolized by the kidney. This means all the inulin that is filtered ends up in the urine, so its clearance equals the glomerular filtration rate. In practice, you infuse inulin, measure its concentration in urine and plasma, and apply the clearance calculation; since there is no tubular handling, the resulting clearance directly reflects GFR. If a substance were secreted, its clearance would be higher than GFR; if reabsorbed, lower; and if metabolized, the measurement would not accurately reflect GFR.