Pharmacokinetics and Pharmacodynamics Practice Questions
20 free Pharmacokinetics and Pharmacodynamics practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following best defines Pharmacokinetics (PK)?
- A What the drug does to the body
- B What the body does to the drug
- C The safety profile of the drug
- D The potency and efficacy of the drug
Correct answer: What the body does to the drug
Pharmacokinetics describes absorption, distribution, metabolism and excretion (ADME) — i.e. what the body does to a drug.
Which parameter describes the theoretical volume that a drug would occupy if it were evenly distributed at the same concentration as in plasma?
- A Clearance
- B Half-life
- C Bioavailability
- D Volume of distribution
Correct answer: Volume of distribution
The apparent volume of distribution (Vd) represents the hypothetical volume into which the drug would distribute to yield the observed plasma concentration.
If a drug’s clearance increases while its volume of distribution remains constant, what happens to its elimination half-life (t½)?
- A Increases
- B Decreases
- C Stays the same
- D Becomes zero
Correct answer: Decreases
Half-life is inversely proportional to clearance (t½ = 0.693 × Vd / CL). Thus increasing clearance shortens half-life if Vd remains constant.
Which route of drug administration yields 100% bioavailability by definition?
- A Oral
- B Sublingual
- C Intramuscular
- D Intravenous
Correct answer: Intravenous
By definition, intravenous administration delivers the drug directly into systemic circulation, giving 100% bioavailability.
Which is most likely when a drug is highly lipophilic and has low plasma protein binding?
- A Low Vd
- B High Vd
- C Rapid clearance
- D No metabolism
Correct answer: High Vd
Lipophilic drugs with low protein binding tend to distribute extensively into tissues, resulting in a high volume of distribution.
What does the term Clearance (CL) refer to in pharmacokinetics?
- A Plasma volume cleared of drug per unit time
- B Fraction of drug bound to plasma proteins
- C Time for plasma concentration to fall by half
- D The total volume of distribution
Correct answer: Plasma volume cleared of drug per unit time
Clearance defines the theoretical volume of plasma cleared of drug per unit time and determines elimination efficiency.
What does Pharmacodynamics (PD) study?
- A Drug absorption and tissue distribution
- B Hepatic drug metabolism pathways
- C Receptor binding and dose-response effects
- D Renal routes of drug excretion
Correct answer: Receptor binding and dose-response effects
Pharmacodynamics studies how the drug affects the body — i.e. receptor binding, post-receptor effects, and dose–response relationships.
A drug reaches steady-state concentration after chronic dosing. Approximately how many half-lives are required to reach ~95% of steady-state plasma levels (assuming first-order kinetics)?
- A 1 half-life
- B 2 half-lives
- C 3–5 half-lives
- D 10 half-lives
Correct answer: 3–5 half-lives
In first-order kinetics, steady state is typically achieved after about 3–5 half-lives when input equals elimination.
Which kinetic model is characterized by a constant rate of elimination, independent of drug concentration?
- A First-order kinetics
- B Zero-order kinetics
- C Mixed-order kinetics
- D Michaelis-Menten kinetics
Correct answer: Zero-order kinetics
Zero-order kinetics eliminates drug at a constant rate regardless of concentration; examples include ethanol and high-dose phenytoin.
Which parameter would most directly determine the loading dose required to rapidly achieve a target plasma concentration?
- A Clearance
- B Fraction of unbound drug in plasma
- C Volume of distribution
- D Oral bioavailability
Correct answer: Volume of distribution
Loading dose equals target plasma concentration multiplied by volume of distribution and divided by bioavailability, so Vd is the parameter that sets how large the initial dose must be. Clearance governs the maintenance dose rather than the loading dose, and protein binding affects Vd only indirectly.
An oral drug undergoes substantial first-pass metabolism in the liver. Which effect does this have on its pharmacokinetics?
- A Increased bioavailability
- B Reduced bioavailability
- C Increased Vd
- D Prolonged half-life
Correct answer: Reduced bioavailability
First-pass metabolism reduces the amount of unchanged drug reaching systemic circulation, thus reducing oral bioavailability.
Which term refers to the fraction of drug unbound to plasma proteins and free to exert effect or be eliminated?
- A Volume of distribution
- B Bioavailability
- C Clearance
- D Free (unbound) fraction
Correct answer: Free (unbound) fraction
Only the unbound fraction (free drug) is pharmacologically active and available for elimination. Protein-bound drug is generally inactive until released.
If two drugs have equal receptor affinity but one elicits a maximal response while the other elicits only a partial response no matter the dose, how are they classified?
- A Full agonist and partial agonist
- B Competitive antagonist and non-competitive antagonist
- C Inverse agonist and agonist
- D Potent agonist and weak agonist
Correct answer: Full agonist and partial agonist
A full agonist produces maximal effect; a partial agonist cannot elicit full effect even at high concentrations despite binding. This is fundamental pharmacodynamic concept.
Which phenomenon describes a decrease in response to a drug over time, requiring higher doses to maintain the same effect?
- A Potentiation
- B Tolerance
- C Synergism
- D Additivity
Correct answer: Tolerance
Tolerance occurs when repeated dosing reduces responsiveness, necessitating higher doses for the same effect — a key pharmacodynamic consideration.
Drug A and Drug B each individually produce 20% reduction in blood pressure. If together they produce 50% reduction, what kind of interaction is this?
- A Antagonism
- B Synergism
- C Additive effect
- D Tolerance
Correct answer: Synergism
Synergism occurs when combined effect is greater than the sum of individual effects (20 + 20 = 40, but observed 50). This is an important PD concept.
Which parameter defines the plasma concentration of drug at which 50% of the receptors are occupied (not necessarily producing 50% of maximal effect)?
- A EC50
- B ED50
- C Kd
- D LD50
Correct answer: Kd
Kd is the equilibrium dissociation constant reflecting affinity; lower Kd means higher affinity. It defines concentration for 50% receptor occupancy, a PD parameter.
If a drug displays first-order elimination kinetics, what happens to the amount eliminated per unit time as plasma concentration decreases?
- A Remains constant
- B Decreases
- C Increases
- D Stops
Correct answer: Decreases
In first-order kinetics elimination is proportional to concentration; thus as concentration decreases, the amount eliminated per unit time decreases.
Which pharmacodynamic parameter refers to the dose at which 50% of the population exhibits a defined therapeutic effect?
- A Kd
- B ED50
- C LD50
- D EC50
Correct answer: ED50
ED50 (effective dose 50) is the dose at which 50% of individuals achieve the desired therapeutic effect. It differs from Kd, which is about receptor binding.
Which pharmacokinetic concept explains why loading doses are used when a drug has long half-life and slow accumulation to steady state?
- A Clearance
- B Volume of distribution
- C First-pass metabolism
- D Pharmacodynamics
Correct answer: Volume of distribution
When Vd is large or half-life is long, drug accumulates slowly; a loading dose based on Vd rapidly achieves therapeutic concentrations before maintenance dosing.
Which factor would most likely prolong the half-life of a drug in an elderly patient?
- A Increased clearance
- B Decreased volume of distribution
- C Reduced renal or hepatic clearance
- D Enhanced protein binding
Correct answer: Reduced renal or hepatic clearance
Reduced organ clearance — e.g., renal or hepatic impairment — slows elimination, prolonging half-life. This affects dosing intervals in older patients.