Question

The plasma drug-concentration versus time curve shown declines such that the half-life remains constant regardless of the starting concentration, and a constant fraction of drug is eliminated per unit time. This pattern best represents which type of elimination kinetics?

• A. First-order kinetics
• B. Zero-order kinetics
• C. Michaelis-Menten (saturation) kinetics at saturating dose
• D. Mixed-order kinetics

Hint:

Constant half-life regardless of dose to exponential, concentration-proportional elimination.

Answer 1

The Correct Option is A

Step 1: Read the curve. A semilog or simple decay curve in which a constant fraction of the drug is removed per unit time, and the half-life (t½) stays the same at high and low concentrations, is the hallmark of first-order kinetics.

Step 2: Apply the concept. In first-order elimination the rate of elimination is proportional to the plasma concentration: as concentration falls, the absolute amount eliminated per unit time also falls, but the proportion (and hence t½) is fixed. Mathematically, \(C = C_0 e^{-kt}\) and \(t_{1/2} = 0.693/k\), independent of \(C_0\).

Step 3: Choose the answer. A constant t½ with concentration-proportional elimination = first-order kinetics, which applies to most drugs at therapeutic doses.

Step 4: Why the others are wrong - zero-order kinetics removes a constant amount per unit time (straight-line linear decline, saturated enzymes, e.g. ethanol, phenytoin, aspirin at high dose) and has no fixed t½; Michaelis-Menten at saturating dose behaves as zero-order; mixed-order shifts from zero- to first-order only as concentration falls below saturation.

Key fact: Constant t½ + concentration-proportional elimination = first-order kinetics (exponential decay).