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USMLE Step 1 Pharmacology: Mechanisms, Toxicities and Antidotes — quiz

12 multiple-choice questions covering Volume of distribution, Half-life and steady state, Dosing adjustments, Zero-order kinetics, Antagonist pharmacodynamics. One answer each; you get a score at the end and the explanation for every question. No account needed.

Updated 2026-11-03. Keys 1–4 pick an answer, Enter moves on. Redo it any time, in order or shuffled.

The questions

Answers and explanations are shown after you finish.

1. Volume of distribution

   A patient receives 300 mg of a drug IV. After distribution, the plasma concentration is 6 mg/L. Calculate the volume of distribution and determine whether the drug is likely confined to plasma or extensively distributed into tissues.

   • 50 L; confined to plasma volume because Vd equals plasma volume
   • 4 L; confined to plasma, consistent with a highly protein-bound drug
   • 6 L; confined to blood volume, typical of an ionized drug
   • 50 L; extensively distributed into tissues, consistent with a lipophilic drug
2. Half-life and steady state

   A drug has a volume of distribution of 40 L and a clearance of 2 L/hr. Approximately how long will it take for plasma levels to reach steady state after starting a continuous infusion?

   • 14 hours
   • 28 hours
   • 7 hours
   • 56-70 hours
3. Dosing adjustments

   A patient with chronic kidney disease is started on a renally cleared drug. Which dosing parameter must be lowered to avoid toxic accumulation, and why?

   • Loading dose, because it depends on volume of distribution, which falls in renal failure
   • Loading dose, because it depends on clearance
   • Maintenance dose, because it depends on volume of distribution
   • Maintenance dose, because it depends on clearance
4. Zero-order kinetics

   A patient on high-dose phenytoin has a plasma concentration that declines in a straight line (constant amount per unit time) after the drug is stopped, rather than declining exponentially. What does this pattern indicate about the elimination kinetics at this dose?

   • Half-life has become constant regardless of dose
   • Clearance has increased because concentration is high
   • First-order kinetics, since a constant fraction is eliminated per unit time
   • Zero-order kinetics, due to saturation of metabolic enzymes at toxic concentrations
5. Antagonist pharmacodynamics

   Adding Drug X to a fixed dose of an agonist shifts the agonist's dose-response curve to the right, but a sufficiently high agonist dose can still achieve the original maximal effect. Drug X is best classified as which type of antagonist?

   • Irreversible antagonist, because it permanently occupies the receptor
   • Partial agonist, because it binds the same receptor
   • Competitive antagonist, because its effect is surmountable with more agonist
   • Noncompetitive antagonist, because it reduces efficacy
6. Partial agonists

   A patient on a full opioid agonist for chronic pain is additionally given a partial opioid agonist. What is the most likely effect on analgesia?

   • Decreased analgesia, because the partial agonist competes for receptors and lowers the net effect
   • Increased risk of respiratory depression from synergistic full agonism
   • Enhanced analgesia from additive agonist effects
   • No change, since partial agonists do not bind the same receptor
7. Potency vs efficacy

   Drug A achieves the same maximal therapeutic effect as Drug B but requires a much lower dose to do so. Which statement correctly describes this relationship?

   • Drug A is more potent than Drug B
   • Drug A is more efficacious than Drug B
   • Drug A has a lower Emax than Drug B
   • Drug B has a higher maximal effect (Emax)
8. Therapeutic index

   Which drug most likely requires close plasma level monitoring because of a narrow margin between its effective and toxic doses?

   • Ibuprofen
   • Penicillin
   • Lithium
   • Amoxicillin
9. Anticholinergic toxicity

   A patient presents with flushed, dry skin, dilated pupils, confusion, urinary retention, and hyperthermia after ingesting an unknown medication. This presentation is most consistent with toxicity from which class of drug?

   • Opioid agonist
   • Sympathomimetic agent
   • Cholinergic (muscarinic) agonist
   • Antimuscarinic (anticholinergic) agent
10. Alpha-blockers

    Why is an irreversible, nonselective alpha-blocker given before surgical removal of a pheochromocytoma?

    • To reverse vasodilation caused by anesthetic agents
    • To prevent a hypertensive crisis from catecholamine release during tumor manipulation
    • To block beta-receptors and prevent reflex tachycardia
    • To decrease catecholamine synthesis within the tumor
11. Digoxin toxicity

    A patient on digoxin develops nausea, yellow-green visual halos, a new arrhythmia, and hyperkalemia. Which mechanism underlies both digoxin's therapeutic effect and this toxicity, and what is the appropriate antidote?

    • Activation of Na+/K+-ATPase, increasing potassium efflux; treat with potassium supplementation
    • Blockade of L-type calcium channels; treat with calcium gluconate
    • Inhibition of Na+/K+-ATPase, raising intracellular calcium via reduced Na+/Ca2+ exchange; treat with digoxin immune Fab
    • Inhibition of phosphodiesterase, increasing cAMP; treat with beta-blockers
12. Antiarrhythmic classes

    A drug that blocks fast sodium channels and is classified as a class IB antiarrhythmic is best represented by which agent?

    • Amiodarone
    • Lidocaine
    • Flecainide
    • Quinidine