Which statement best describes zero-order kinetics?

Which statement best describes zero-order kinetics?

• A. A constant amount of drug is removed per unit time
• B. A constant fraction of drug is removed per unit time
• C. The amount removed per unit time increases with concentration
• D. Concentration decays exponentially

Answer: (A)

In zero-order kinetics, the body eliminates a constant amount of drug per unit time because the elimination pathway is saturated. This means the rate of elimination does not depend on how much drug is present—the same amount is removed each hour (or other time unit) as long as saturation holds. As a result, the amount in the body declines linearly over time (until the saturating mechanism is no longer engaged). This is captured by the statement that a constant amount of drug is removed per unit time. The other descriptions describe different patterns: removing a constant fraction per unit time corresponds to first-order kinetics, where the percentage, not the absolute amount, is constant and the concentration decays exponentially. Saying the amount removed per unit time increases with concentration would imply a rate that speeds up as concentration rises, which is not zero-order. Concentration decaying exponentially is the hallmark of first-order kinetics, not zero-order.

In zero-order kinetics, the body eliminates a constant amount of drug per unit time because the elimination pathway is saturated. This means the rate of elimination does not depend on how much drug is present—the same amount is removed each hour (or other time unit) as long as saturation holds. As a result, the amount in the body declines linearly over time (until the saturating mechanism is no longer engaged).

This is captured by the statement that a constant amount of drug is removed per unit time. The other descriptions describe different patterns: removing a constant fraction per unit time corresponds to first-order kinetics, where the percentage, not the absolute amount, is constant and the concentration decays exponentially. Saying the amount removed per unit time increases with concentration would imply a rate that speeds up as concentration rises, which is not zero-order. Concentration decaying exponentially is the hallmark of first-order kinetics, not zero-order.