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Pharmacokinetics: Absorption, Distribution, Metabolism, and Excretion MCQs
Notes · Pharm-D / Pharmacy · 18 practice questions
- P-glycoprotein Drug Interactions
- CYP3A4 Interactions
- Excretion and Urinary pH
- Loading Dose Calculation
- Volume of Distribution
- Half-Life, Clearance, and Volume of Distribution Relationships
- Blood-Brain Barrier Penetration
- Absorption: P-glycoprotein and Ionization
1. A patient takes aspirin (a weak acid) with a large meal that significantly delays gastric emptying and raises gastric pH transiently. Based on the principles of non-ionic diffusion, which statement best predicts the effect on aspirin absorption in the stomach during this period?
- AAbsorption increases because P-glycoprotein efflux is inhibited by acidic pH
- BAbsorption increases because more drug becomes ionized and diffusible
- CAbsorption is unaffected because aspirin is absorbed exclusively in the small intestine
- DAbsorption decreases because more drug becomes ionized, reducing lipid membrane diffusion
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D. Absorption decreases because more drug becomes ionized, reducing lipid membrane diffusion Weak acids are predominantly un-ionized (and thus better absorbed) in acidic environments; raising gastric pH would increase ionization of the weak acid aspirin, reducing its passive diffusion across the lipid membrane.
2. Vecuronium, a quaternary ammonium neuromuscular blocker, acts at peripheral cholinergic synapses but produces no central nervous system effects. Which property of the drug best explains this lack of CNS penetration?
- AIt is a substrate for P-glycoprotein efflux at the blood-brain barrier
- BIt has a very small volume of distribution, confining it to plasma
- CIt is permanently ionized and therefore cannot cross the lipid blood-brain barrier
- DIt undergoes rapid first-pass hepatic metabolism before reaching the brain
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C. It is permanently ionized and therefore cannot cross the lipid blood-brain barrier The blood-brain barrier restricts ionized/polar molecules; quaternary ammonium compounds are permanently charged, preventing lipophilic diffusion needed for CNS penetration, unlike drugs requiring lipophilicity and non-ionization at physiological pH.
3. A patient with acute atrial fibrillation and normal renal/hepatic function requires rapid therapeutic digoxin levels, while a second patient with chronic kidney disease is being maintained on digoxin long-term. Which statement correctly reflects appropriate dosing calculations for these two scenarios?
- AThe loading dose should be reduced in the CKD patient because clearance is reduced, while maintenance dose remains unaffected
- BBoth loading and maintenance doses should be reduced in the CKD patient because both depend on clearance
- CThe loading dose in the first patient depends on Vd and target concentration only, while the maintenance dose in the CKD patient must be reduced in proportion to reduced clearance
- DBoth loading and maintenance doses are calculated identically using clearance and target concentration
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C. The loading dose in the first patient depends on Vd and target concentration only, while the maintenance dose in the CKD patient must be reduced in proportion to reduced clearance Loading dose = Vd × target concentration, independent of clearance, while maintenance dose = CL × target concentration × dosing interval, and must be reduced in proportion to reduced clearance in renal or hepatic impairment.
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