Under Michaelis-Menten (nonlinear) kinetics, how does clearance behave at high drug concentrations?

• A. Clearance remains constant
• B. Clearance increases with concentration
• C. Clearance declines toward Vmax/Km
• D. Clearance becomes zero

Answer: (C)

Under Michaelis-Menten (nonlinear) elimination, what can be removed per unit time cannot exceed Vmax, and the observed clearance changes with concentration. The rate equation is v = (Vmax × C) / (Km + C). Clearance is defined as CL = v / C, which simplifies to CL = Vmax / (Km + C).

This shows how clearance behaves as concentration changes. At high concentrations (C much larger than Km), the denominator is dominated by C, so CL ≈ Vmax / C. That means clearance falls as concentration rises and, in the limit of very high concentrations, it tends toward zero. In contrast, at very low concentrations (C near zero), CL approaches Vmax / Km, which is a finite, maximum clearance value in the saturable system.

So the key idea is that clearance is not constant in nonlinear kinetics—it decreases with increasing drug concentration due to enzyme saturation. The high-concentration behavior is a decline toward zero, while the low-concentration behavior is a finite ceiling near Vmax/Km.