Inulin clearance is the gold standard for GFR measurement. A patient has a plasma inulin concentration of 1 mg/mL and is excreting inulin in urine at 120 mg/min. What is his GFR?

• A 60 mL/min
• B 180 mL/min
• C 120 mL/min ✓
• D 240 mL/min

Explanation

GFR (inulin clearance) = Urine inulin excretion / Plasma inulin concentration = 120 mg/min ÷ 1 mg/mL = 120 mL/min. Inulin is freely filtered, not reabsorbed, not secreted, and not synthesised by the kidney, making its clearance exactly equal to GFR. Normal GFR in a 70 kg adult is approximately 125 mL/min (180 L/day). Creatinine clearance slightly overestimates GFR because creatinine is also secreted by the proximal tubule (~10–20% contribution).

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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