A patient is given inulin intravenously and reaches a steady plasma concentration of 1.5 mg/mL. Urine inulin concentration is 150 mg/mL, and urine flow rate is 1 mL/min. The calculated GFR is:

• A 150 mL/min
• B 100 mL/min ✓
• C 225 mL/min
• D 50 mL/min

Explanation

GFR is calculated as the clearance of inulin, which is freely filtered and neither reabsorbed nor secreted. Clearance = (Urine concentration × Urine flow rate) / Plasma concentration = (150 mg/mL × 1 mL/min) / 1.5 mg/mL = 150/1.5 = 100 mL/min. This is normal GFR for an average adult. Inulin clearance is the gold standard for measuring GFR. Creatinine clearance slightly overestimates GFR because creatinine is also secreted by the proximal tubule (~10–20% of urinary creatinine is secreted).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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