A concentration-time curve that starts at a high concentration and declines monotonically is most indicative of which route of administration?

• A. Oral administration
• B. IM injection
• C. IV bolus
• D. IV constant infusion

Answer: (C)

A concentration–time curve that starts at a high plasma concentration and then declines steadily without an initial rise reflects direct entry into the bloodstream with no absorption phase. This is characteristic of an IV bolus dose. Right after administration, the drug concentration is essentially immediately high (often called C0), and the subsequent decline is due to distribution into tissues and then elimination from the body, typically following first-order kinetics (a monoexponential fall).

In contrast, routes that involve absorption—oral or intramuscular injections—always include a phase where the drug is moving from the site of administration into the bloodstream. That means the curve usually rises to a peak (Cmax) after some time (Tmax) before it declines, not starting at a high concentration right away. An IV constant infusion dose, on the other hand, causes the concentration to rise during the infusion until it reaches a steady-state, rather than starting high and monotonically decreasing from the outset.

So the best match for a curve that is high at the start and monotonically declines is an IV bolus administration, where there’s immediate systemic exposure and subsequent elimination-driven decay.