Pharmaceutics Drug Disposition Practice Test

How does the elimination rate change as concentration increases for first-order and zero-order kinetics?

First-order: elimination rate increases with concentration; Zero-order: elimination rate is constant First-order: elimination rate constant; Zero-order: elimination rate increases with concentration First-order: elimination rate is constant; Zero-order: elimination rate decreases with concentration First-order: elimination rate decreases with concentration; Zero-order: elimination rate increases

Correct Answer:

First-order: elimination rate increases with concentration; Zero-order: elimination rate is constant

Explanation:

In first-order processes, the amount eliminated per unit time is proportional to how much drug is present. That means increasing the concentration directly increases the elimination rate, while the rate constant (which sets the proportionality) stays the same regardless of concentration. In zero-order processes, the system is saturated, so a fixed amount is eliminated per unit time regardless of how much drug there is—so the elimination rate stays constant until the concentration drops enough to fall out of saturation. Therefore, as concentration rises, first-order elimination rate increases, and zero-order elimination rate remains constant. This matches the statement that first-order rate increases with concentration and zero-order rate is constant.

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