Inulin clearance is measured to estimate-

Ans. is 'd' i.e., GFR * If there be a substance that passes freely across the glomerular membane but is neither reabsorbed nor secreted by tubular activity, urinary excretion would represent the amount filtered and nothing but the amount filtered. Hence, its clearance would indicate how much plasma gets filtered every minute.* One such substance is Inulin, a polysaccharide of small enough (molecular weight about 5,200) to get filtered as easily as crystalloids or water. It is neither reabsorbed nor secreted by the kidney. Further, it is neither metabolized nor synthesized by the kidneys, is non-toxic and can be easily measured. All these features make it a very suitable substance for estimation of glomerular filtration rate. Inulin clearance is equal to the GFR, i.e., 126 ml/min.* Beside inulin, the clearance of vitamin B12, EDTA, and polyfructosan are also quite reliable as indicators of GFR Creatinine is not as accurate as the above substances because it is secreted in small amounts. But since it is a product of metabolism and its level is fairly constant, creatinine is very convenient to determine its clearance. That is why, clinically, creatinine clearance is used an index of GFR quite frequently.Remember* Inulin is more accurate measure of GFR, but is inconvenient because it has to be injected intravascular.* Creatinine is not as accurate as inulin, But creatinine is already present in the blood in steady-state, so the endogenous creatinine clearance can be used without the necessity of having to infuse inulin.