**Question:** 100% Bioavailability is seen with this route:

A. Injection (intravenous, intramuscular, or subcutaneous)
B. Inhalation
C. Oral
D. Topical

**Correct Answer:** .

**Core Concept:**
Bioavailability is a pharmacokinetic parameter that describes how much of a drug enters the systemic circulation from a given dose. There are four types of bioavailability:
1. Fraction absorbed (Fa): The fraction of the drug that is absorbed from the site of administration.
2. Absolute bioavailability (Fa x Fa): A measure of the total amount of the drug that enters the systemic circulation.
3. Relative bioavailability: A comparison of the bioavailability of two formulations of the same drug.
4. Bioequivalence: The bioavailability of two formulations of a drug is considered equivalent if the 90% confidence interval of the ratio of the two bioavailability values falls within a pre-defined range (80-125% for FDA, 80-125% for EMA).

**Why the Correct Answer is Right:**

100% bioavailability is associated with the intravenous (IV), intramuscular (IM), or subcutaneous (SC) routes of administration. In these routes, the drug is directly injected into a blood vessel, bypassing the first-pass effect of the liver, which is responsible for approximately 90% of the drug being destroyed or altered before reaching the systemic circulation. This results in a much higher fraction absorbed (Fa) and absolute bioavailability (Fa x Fa) compared to other administration routes.

**Why Other Options are Incorrect:**

- **Option A (Inhalation):** Bioavailability of inhaled drugs depends on factors such as particle size, deposition in the lungs, and drug solubility. Inhalation bioavailability is generally lower than 100% due to these factors.
- **Option B (Topical):** Topical drugs are absorbed through the skin, which is a semi-permeable barrier. The absorption rate and extent are influenced by factors like drug concentration, pH, and the presence of skin impurities. Bioavailability in this route is usually <100%. - **Option C (Oral):** Bioavailability of orally administered drugs depends on factors like gastric pH, enzymatic degradation, and first-pass effect. Bioavailability is generally <100%, particularly for highly lipophilic and large molecules. **Clinical Pearl:** In clinical practice, intravenous administration is often preferred when rapid onset of action is required, as it provides immediate drug delivery to the systemic circulation, bypassing the first-pass effect of the liver. However, oral administration is generally more practical for routine patient care due to its convenience and ease of use. When considering drug selection, the available form, patient preference, and disease state should be taken into account.