What factors influence a medication's bioavailability?

The correct factor that influences a medication's bioavailability is the route of administration, first-pass metabolism, and drug formulation.

Bioavailability refers to the proportion of a drug that enters the circulation when introduced into the body and is available for action. The route of administration significantly affects how much of the drug is absorbed. For example, oral medications may undergo considerable first-pass metabolism, where the drug is metabolized by the liver before it reaches systemic circulation, thus reducing the bioavailability. In contrast, drugs administered intravenously bypass the digestive system and liver, leading to 100% bioavailability.

Additionally, drug formulation plays a crucial role because the physical and chemical properties of the drug (like solubility and stability) and how it's formulated (tablet, liquid, etc.) can affect absorption rates and the extent to which the drug is available in the bloodstream.

While patient factors, such as age, weight, health, and duration of medication use, can influence overall treatment effectiveness and how a patient responds to a drug, they do not directly determine the inherent pharmacokinetic properties of bioavailability. Thus, the specified factors—route of administration, first-pass metabolism, and drug formulation—are fundamental determinants of a medication's bioavailability.