In a one-compartment IV bolus model, which expression describes the plasma concentration C(t) as a function of time?

• A. C(t) = (Dose / Vd) × e^{-kt}
• B. C(t) = Dose / (Vd × e^{kt})
• C. C(t) = (Vd / Dose) × e^{kt}
• D. C(t) = Dose × e^{-kt} / Vd

Answer: (A)

In a one-compartment IV bolus, the drug is instantly distributed within the apparent volume of distribution, Vd. Right after administration, the plasma concentration is Dose/Vd. After that moment, elimination follows first-order kinetics, so the concentration decays exponentially with the rate constant k. This gives the expression C(t) = (Dose/Vd) e^{-kt}: the initial concentration is Dose/Vd and the subsequent decline is governed by e^{-kt}. The half-life relationship t1/2 = 0.693/k explains how long it takes for the concentration to drop by half. This form cleanly links the starting level and the exponential decay, which is why it is the standard expression for C(t) in this model.