A curved negative slope in a concentration-time plot indicates which type of elimination kinetics?

• A. Zero-Order Elimination Kinetics
• B. Mixture of orders
• C. First-Order Elimination Kinetics
• D. No elimination

Answer: (C)

The main idea is that the shape of a concentration–time plot reveals the elimination order: a curved, negatively sloped curve shows that the rate of elimination changes as concentration falls, which is characteristic of first-order elimination. In first-order kinetics, the amount eliminated per unit time is proportional to the remaining concentration, so the drug decays exponentially and the plot bends downward rather than forming a straight line. If you plotted the natural log of concentration versus time, you’d get a straight line with slope -k, illustrating the constant proportional rate. In contrast, zero-order elimination would produce a straight line with a constant negative slope because a fixed amount is eliminated per unit time, regardless of concentration. A mixture of orders or no elimination would not produce this clean curved negative slope.