Define absolute and relative bioavailability.

• A. Absolute compares two non-IV routes or formulations; relative compares extravascular to IV.
• B. Absolute compares intravascular to extravascular; relative compares two non-IV routes.
• C. Absolute bioavailability always equals 1.
• D. Absolute compares extravascular to IV; relative compares two non-IV routes or formulations.

Answer: (D)

The main idea is how bioavailability is quantified against a reference route. Absolute bioavailability measures what fraction of a non-intravenous dose actually reaches the systemic circulation by comparing that route to an intravenous dose (where the drug is 100% available). In practice this is calculated from AUCs and doses: F = (AUC_extravascular / Dose_extravascular) ÷ (AUC_IV / Dose_IV). Relative bioavailability, by contrast, compares two non-intravenous routes or two formulations to each other, to see how much more or less of the drug is absorbed between them, often using a ratio of AUCs adjusted for dose. So the correct statement is that absolute compares extravascular to IV, while relative compares two non-IV routes or formulations. Remember, absolute is not always 1; that would only be true for the IV reference itself, not for extravascular routes.