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Ans: a (Clearance) Ref: Ganong,22nd ed, p. 705Clearance of a substance is equal to the concentration of that substance in urine multiplied by urine flow rate divided by arterial plasma level, i.e.Clearance=UxV----PxUx - cone of substance in urineV -urine flow ratePx - cone of substance in plasmaNote:Inulin is used to measure GFR because it is freely filtered and neither reabsorbed nor secreted in the tubules. But in practice we use creatinine clearance to measure GFR because it is easier to measure.PAHA is used to measure renal plasma flow i.e.Clearance of para-aminohippuric acid is equal to effective renal plasma flow.From the renal plasma flow, the renal blood flow can be calculated by dividing by 1 minus the haematocritRenal blood flow = RPF x1------------(1-Haematocrit)Renal blood flow = 1273ml/min which account for about 25% of cardiac output.