Which statement best distinguishes zero-order from first-order elimination?

• A. Zero-order elimination rate is proportional to concentration.
• B. Zero-order elimination rate increases with time.
• C. First-order elimination has a constant elimination rate regardless of concentration.
• D. Zero-order elimination has a constant elimination rate regardless of concentration while first-order elimination rate is proportional to concentration.

Answer: (D)

The key idea is how elimination rate depends on drug concentration. In zero-order elimination the body removes a constant amount per unit time, regardless of how much drug is present. This happens when the elimination pathways are saturated and can’t speed up as concentration rises. In first-order elimination, the rate of removal is proportional to how much drug is in the body—more drug means a faster elimination rate, leading to exponential decay and a constant half-life.

The statement that zero-order elimination has a constant rate regardless of concentration while first-order elimination rate is proportional to concentration captures exactly this distinction. It describes the zero-order behavior (rate independent of concentration) and the first-order behavior (rate proportional to concentration).

The other options misstate the relationships: zero-order being proportional to concentration is characteristic of first-order, and first-order having a constant rate regardless of concentration would describe zero-order. Saying zero-order rate increases with time isn’t accurate for the rate itself, which remains constant (though the amount removed over time remains the same).