Pharmacology Fundamentals Flashcards

Which pharmacokinetic parameter describes the time for plasma concentration to fall by 50% after distribution equilibrium?

Answer: Context-sensitive half-time

Context-sensitive half-time reflects the time for plasma concentration to halve after stopping an infusion, and varies with infusion duration unlike a simple elimination half-life.

A patient on phenytoin requires an increased dose of vecuronium. The most likely pharmacokinetic explanation is:

Answer: Induction of hepatic CYP450 enzymes increasing vecuronium metabolism

Phenytoin is a potent CYP450 inducer that accelerates hepatic metabolism of vecuronium, reducing its plasma concentration and duration of action.

Which statement best describes zero-order kinetics?

Answer: A constant amount of drug is eliminated per unit time regardless of concentration

Zero-order kinetics occurs when elimination pathways are saturated, so a fixed amount (not fraction) of drug is cleared per unit time.

The loading dose of a drug is primarily determined by:

Answer: Volume of distribution and target plasma concentration

Loading dose = (Volume of distribution × Target plasma concentration) / Bioavailability, making Vd the dominant determinant.

Which receptor type mediates the inotropic effects of dopamine at doses of 5–10 mcg/kg/min?

Answer: Beta-1 adrenoceptors

At intermediate doses (5–10 mcg/kg/min), dopamine predominantly stimulates beta-1 adrenoceptors, increasing heart rate and contractility.

Which property of a drug most influences its ability to cross the blood-brain barrier?

Answer: Molecular weight below 500 Da and high lipid solubility

Drugs penetrate the BBB most readily when they are small, lipophilic, and largely unionised at physiological pH.

A patient with hepatic failure is prescribed a drug with high hepatic extraction ratio (E > 0.7). What is the expected pharmacokinetic consequence?

Answer: Markedly increased oral bioavailability due to reduced first-pass effect

High extraction ratio drugs undergo extensive first-pass hepatic metabolism, so liver failure dramatically increases their systemic bioavailability after oral dosing.