<prompt> Ideal substance for measuring GFR should have all characteristics exceptShould be secreted in tubulesShould be non-toxicShould not remain in the bodyShould not be protein bound  <response>: The Ideal substance to measure GFR is Inulin or following reasons ; It is non-toxic; It is freely filterable by the glomeruli; it is not bound to proteins It is neither secreted nor reabsorbed by the tubules; It is not synthesized, stored, or destroyed by the kidney . Thus, Filtered inulin excreted inulin .Hence, it will give an exact estimate of the GFR. Endogenous creatinine clearance is used most commonly used to estimate the GFR. 