Tolerance is a diminished response to the same dose of a drug after repeated exposure. It can develop slowly through pharmacodynamic changes, such as receptor downregulation or desensitization, or acutely as tachyphylaxis. This differs from acquired resistance (often seen in microbes or cancer cells becoming unresponsive to a drug), sensitization (an increased response with exposure), and potentiation (one drug increasing the effect of another without itself becoming less effective).

Area under the concentration–time curve represents the total amount of drug that reaches the systemic circulation over time, i.e., the overall systemic exposure. When exposure is larger, more drug is present in the body across the dosing interval, which is why a bigger AUC corresponds to greater systemic exposure. In linear pharmacokinetics, AUC equals F × Dose divided by clearance, so for a given dose and clearance, increasing AUC means more drug has entered and remained in the body. Absorption rate mainly changes how fast the drug enters (affecting peak level and time to peak), not the total amount that reaches circulation. The peak SAMPLEconcentration is about Cmax, not the total exposure. Half-life reflects how long the drug stays in the body, but AUC is governed by dose, bioavailability, and clearance.

Zero-order elimination means the body removes a constant amount of drug per unit time, regardless of how much drug is present. This happens when the elimination process is saturated, so capacity limits the rate rather than concentration. Because the rate is fixed, it does not rise as concentration increases, and the concentration declines linearly over time until the drug is depleted. That’s why the statement describing the rate as constant and independent of concentration is the best fit. In zero-order kinetics, half-life is not constant across concentrations, since the amount removed per unit time is fixed. The idea that the rate increases with concentration is incorrect because the rate does not depend on concentration. While clearance would appear to increase over time as concentration falls (rate constant but concentration decreasing), the defining feature examiners focus on is the constant elimination rate itself.

How the concentration of a drug falls over time depends on the order of elimination. In first-order elimination, the removal rate is proportional to how much drug is present, so the concentration-time profile follows an exponential decay: C = C0 e^{-kt}. This means the curve is curved—steepest at the start and then it gradually levels off as time goes on. In zero-order elimination, the drug is removed at a constant amount per unit time, so the concentration declines linearly: C = C0 − k0 t. On a concentration-versus-time graph, this is a straight line with a constant negative slope, at least until the drug concentration hits zero or the elimination mechanism changes. Therefore, the correct description is that first-order elimination shows an exponential decline, while zero-order elimination shows a straight-line decline.