Plasma concentration does not correlate with response in single-dose therapy.

• A. Not enough information
• B. Sometimes
• C. False
• D. True

Answer: (D)

Plasma concentration and pharmacologic response are not always in step, especially after a single dose. The effect of a drug depends on the amount that actually reaches the site of action, which can differ from what’s circulating in the blood. Distribution into tissues, binding to components, and the time required for the drug to interact with receptors create a lag between plasma levels and the observed effect. This delay means you can have high plasma concentrations without an immediate or proportional response, or conversely, an effect that persists even as plasma levels fall. Because single-dose therapy doesn’t reach steady-state distribution, the relationship between plasma level and response is often imperfect, making the statement true.