Which statement correctly characterizes first-order elimination kinetics?

• A A constant amount of drug is eliminated per unit time regardless of plasma concentration
• B The plasma concentration declines linearly with time on an arithmetic plot
• C The rate of elimination is proportional to the plasma concentration, so a constant fraction is removed per unit time ✓
• D Half-life shortens progressively as concentration falls

Explanation

First-order kinetics means the elimination rate is proportional to concentration, so a fixed fraction of the drug present is cleared per unit time and the half-life remains constant at all concentrations. On an arithmetic plot the decline is exponential, becoming linear only on a semilogarithmic plot. Option A describes zero-order kinetics, where a constant amount is removed per unit time and half-life varies with concentration, as seen with ethanol and phenytoin at saturating doses.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

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