Para-aminohippuric acid (PAH) is infused until its plasma concentration is well below its transport maximum. Its clearance at this point is used clinically as a measure of which variable?

• A Glomerular filtration rate
• B Free water clearance
• C Filtration fraction
• D Effective renal plasma flow ✓

Explanation

PAH is filtered at the glomerulus and then almost completely cleared from the peritubular capillaries by organic anion secretion in the proximal tubule, so at low plasma concentrations its extraction by the kidney approaches 90 percent. Its clearance therefore estimates effective renal plasma flow, which excludes flow to juxtamedullary regions that are less well extracted. Dividing GFR by this clearance yields the filtration fraction.

Reference: Vander's Renal Physiology, 8th ed.

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