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Clearance

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We can calculate the renal clearance of any substance if we know which pair of values?

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a. Urine flow rate and urine concentration

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b. Plasma concentration and urine concentration

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c. GFR and urinary excretion rate

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d. Plasma concentration and urinary excretion rate

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(d) The excretion rate of a substance divided by its plasma concentration yields the clearance.

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A drug X has a short plasma half-life and must be administered frequently to maintain therapeutic levels. The urinary concentration of X is much higher than the plasma concentration. A substantial amount of X also appears in the feces. What can we say about the renal clearance of X compared with the metabolic clearance rate of X?

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a. The metabolic clearance rate is higher than the renal clearance.

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b. The renal clearance is higher than the metabolic clearance rate.

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c. The 2 clearances are the same.

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d. There is insufficient information to answer the question.

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(a) The metabolic clearance rate represents the sum of all clearance routes. Since there are 2 major routes of clearance (kidneys and feces) the metabolic clearance must be higher than either one alone. The fact that the drug has a higher urinary concentration than plasma concentration mainly reflects the reabsorption of water.

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Inulin clearance is measured twice: the first time at a low inulin infusion rate, and the second time at a higher infusion rate that results in a higher plasma inulin concentration during the test. Assuming the kidneys behave the same in both cases, which measurement will yield a higher inulin clearance?

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a. The first

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b. The second

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c. Both measurements are the same

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d. There is insufficient information to answer the question

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(c) In the second test both the plasma concentration and filtered load (and hence rate of excretion) are increased, yielding offsetting effects on the calculation.

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Which of the following indicates correct relative renal clearances?

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a. Sodium clearance is greater than urea clearance.

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b. PAH clearance is greater than inulin clearance.

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c. Urea clearance is greater than PAH clearance.

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d. Creatinine ...

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