Inulin clearance in a patient measures 90 mL/min while simultaneous creatinine clearance measures 105 mL/min. Which property of creatinine best accounts for this discrepancy?

• A Creatinine is partially reabsorbed by the proximal tubule
• B Creatinine has a lower glomerular sieving coefficient than inulin
• C Creatinine is secreted by the proximal tubule via organic cation transporters ✓
• D Creatinine undergoes extrarenal degradation that lowers its plasma concentration

Explanation

Creatinine is small and freely filtered like inulin, but it is also actively secreted by proximal tubular cells through organic cation transporters. This additional secretion adds to the filtered load, making urinary creatinine exceed what filtration alone would deliver, so creatinine clearance systematically overestimates GFR by roughly 10 to 20 percent. Reabsorption would produce the opposite error, and the sieving coefficient of creatinine is essentially 1, identical to inulin. Extrarenal gut degradation slightly lowers serum levels and further inflates the apparent clearance.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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