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GFR Measurement — FRCA Primary MCQ

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HardPhysiology - RenalGFR MeasurementFRCA Primary

At steady state, a marker has urine concentration 12 mg/mL, plasma concentration 0.10 mg/mL and urine flow 1.0 mL/min. It is freely filtered and neither reabsorbed nor secreted. What physiological variable does this measurement estimate?

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Correct answer: B — Glomerular filtration rate of 120 mL/min

The best answer is “Glomerular filtration rate of 120 mL/min”. Clearance is U × V/P, so 12 × 1.0/0.10 = 120 mL/min. A freely filtered marker with neither tubular reabsorption nor secretion has a clearance equal to GFR. Renal plasma flow requires a highly extracted marker such as PAH, and filtration fraction is a dimensionless ratio of GFR to renal plasma flow.

Reference: RCoA, Primary FRCA syllabus: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf; Measurement of renal function in chronic kidney disease: https://pmc.ncbi.nlm.nih.gov/articles/PMC3791974/