A substance X has a clearance of 700 mL/min in a healthy adult. Plasma concentration of X is 1 mg/mL. GFR is 120 mL/min. Which statement about substance X is correct?
- A X is completely reabsorbed
- B X clearance cannot exceed GFR, so the data is erroneous
- C X is filtered and secreted; renal plasma flow is likely around 700 mL/min ✓
- D X is synthesized by tubular cells
Correct answer: C. X is filtered and secreted; renal plasma flow is likely around 700 mL/min
Explanation
Clearance exceeding GFR indicates net tubular secretion. A clearance near 700 mL/min approaches renal plasma flow, similar to PAH. The upper limit of clearance equals renal plasma flow (about 650 mL/min in a healthy adult). Inulin clearance equals GFR; substances with clearance above GFR are secreted.
Reference: BRS Physiology, 7th ed.
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