Which statement differentiates first-order from zero-order absorption?

• A. First-order absorption rate is proportional to the amount remaining, zero-order absorption is a constant amount per unit time.
• B. First-order absorption is a constant amount per unit time, zero-order is proportional to remaining amount.
• C. Both are proportional to remaining amount.
• D. Both are constant over time.

Answer: (A)

The key idea is how the absorption rate depends on how much drug is still available to be absorbed. In first-order absorption, the rate into systemic circulation is proportional to the amount remaining in the absorption site—more drug left means a faster absorption, and as that amount decreases, the rate slows down. In zero-order absorption, the drug enters at a constant amount per unit time, regardless of how much is left, until depletion.

This matches the statement that first-order is proportional to the amount remaining, while zero-order is a constant amount absorbed per unit time. The other descriptions don’t fit: first-order is not a fixed amount per time, zero-order is not proportional to the remaining amount, and you don’t have both processes being constant or both proportional.