Pharmacology Pharmacokinetics — Questions and Answers
Question 1: Which of the following processes is primarily responsible for the movement of a drug from its site of administration into the bloodstream?
- Distribution
- Absorption (Correct answer)
- Metabolism
- Excretion
Correct answer: Absorption
Absorption is the process by which a drug moves from its site of administration (e.g., gastrointestinal tract, skin, muscle) into the systemic circulation (bloodstream). This is the first step in pharmacokinetics, determining how much of the drug actually enters the body to exert its effects. Distribution, metabolism, and excretion occur after absorption.
Question 2: The term "bioavailability" refers to:
- The amount of drug absorbed and available at the target tissue
- The percentage of the administered dose that reaches systemic circulation unchanged (Correct answer)
- The volume of distribution of the drug
- The rate of drug elimination from the body
Correct answer: The percentage of the administered dose that reaches systemic circulation unchanged
Bioavailability is a pharmacokinetic term that quantifies the fraction of an administered drug dose that reaches the systemic circulation in an unchanged, active form. It is a crucial factor in determining the appropriate dosage and route of administration, as it reflects how much of the drug is actually available to produce its therapeutic effect. A drug with low bioavailability may require a higher dose or a different administration route.
Question 3: Which organ is most responsible for the metabolism of drugs?
- Kidneys
- Liver (Correct answer)
- Stomach
- Lungs
Correct answer: Liver
The liver is the primary organ responsible for drug metabolism, a process that converts drugs into more water-soluble compounds for easier excretion. It contains a vast array of enzymes, particularly the cytochrome P450 (CYP) system, which chemically modify drugs. While other organs contribute, the liver's role is dominant in detoxifying and processing xenobiotics.
Question 4: What does "first-pass metabolism" refer to?
- The process of drug excretion through the kidneys
- The rapid metabolism of a drug in the stomach
- The metabolism of a drug by the liver before it reaches systemic circulation (Correct answer)
- The binding of a drug to plasma proteins
Correct answer: The metabolism of a drug by the liver before it reaches systemic circulation
First-pass metabolism, or pre-systemic metabolism, refers to the phenomenon where a significant portion of an orally administered drug is metabolized by the liver and/or gut wall enzymes before it reaches the systemic circulation. This reduces the amount of active drug available to exert its therapeutic effect, often necessitating higher oral doses compared to intravenous administration. It's a key consideration in drug formulation and dosage.
Question 5: A drug with a half-life of 4 hours will have approximately 25% of the initial dose remaining in the body after:
- 2 hours
- 4 hours
- 8 hours (Correct answer)
- 12 hours
Correct answer: 8 hours
A drug's half-life is the time it takes for the amount of the drug in the body to reduce by half. After one half-life (4 hours), 50% of the initial dose remains. After a second half-life (another 4 hours, totaling 8 hours), the remaining 50% is halved again, leaving 25% of the initial dose.
Pharmacology Pharmacokinetics
Which of the following processes is primarily responsible for the movement of a drug from its site of administration into the bloodstream?