The rate of drug elimination by the kidney at 1000x concentration is the same as at 1x concentration.

• A. Lower at 1000x
• B. Higher at 1000x
• C. Cannot be determined
• D. Same at 1000x

Answer: (D)

The rate of renal elimination can become saturable when the drug is handled by active tubular transporters. These processes follow Michaelis-Menten kinetics, where rate = (Vmax × [S]) / (Km + [S]). When the drug concentration is far above Km, the transporters are saturated and the rate approaches a maximum value, Vmax, becoming almost independent of further increases in concentration. So increasing from 1x to 1000x does not raise the elimination rate because it’s already at or near Vmax. This is why the kidney’s elimination rate can be the same at very high concentrations as at normal concentrations, assuming transporter-mediated clearance dominates.