**Core Concept**
Bioavailability refers to the fraction of an administered dose of unchanged drug that reaches the systemic circulation and is one of the principal pharmacokinetic properties of drugs. It is a critical parameter in pharmacology that determines the effectiveness and efficacy of a drug. Bioavailability is influenced by various factors such as absorption, first-pass metabolism, and formulation of the drug.

**Why the Correct Answer is Right**
The correct answer, "Amount of drug that reach the systemic circulation," accurately describes bioavailability. Bioavailability is a measure of the extent to which a drug is absorbed and reaches the systemic circulation in its active form. It is typically expressed as a percentage of the administered dose. For example, if a drug has a bioavailability of 80%, it means that 80% of the administered dose reaches the systemic circulation. This is in contrast to the other options, which either describe different pharmacokinetic properties or are incorrect statements.

**Why Each Wrong Option is Incorrect**
**Option B:** This option describes first-pass metabolism, which is a process where a drug is metabolized in the liver before it reaches the systemic circulation. While first-pass metabolism can affect bioavailability, it is not a definition of bioavailability itself.

**Option C:** This option is incorrect because it describes drug metabolism that occurs after the drug reaches the systemic circulation, which is not a definition of bioavailability. Bioavailability is concerned with the amount of drug that reaches the systemic circulation, not the amount that is metabolized after it reaches the circulation.

**Option D:** This option is incorrect because maximum bioavailability is not limited to the rectal route. While the rectal route can have high bioavailability for certain drugs, it is not a universal rule that maximum bioavailability is achieved by this route.

**Clinical Pearl / High-Yield Fact**
When evaluating the bioavailability of a drug, it is essential to consider factors such as absorption, first-pass metabolism, and formulation. A high bioavailability does not necessarily translate to a high therapeutic effect, as other factors such as receptor binding and efficacy also play a crucial role.

**Correct Answer Line**
✓ Correct Answer: A. Amount of drug that reach the systemic circulation