Pharmacokinetics
Help Questions
USMLE Step 1 › Pharmacokinetics
A 48-year-old man with cirrhosis from nonalcoholic steatohepatitis is started on oral propranolol for portal hypertension prophylaxis. He develops lightheadedness and bradycardia after the first few doses. Exam: HR 48/min, BP 92/54 mm Hg. Labs: total bilirubin 3.1 mg/dL, albumin 2.8 g/dL, INR 1.7, creatinine 1.0 mg/dL. Propranolol undergoes extensive first-pass hepatic metabolism with low oral bioavailability in healthy adults; Vd $\approx 4\ \text{L/kg}$ and half-life $\approx 3$–6 hours. In cirrhosis, first-pass metabolism is reduced, increasing systemic exposure after oral dosing. Which of the following best describes how the pharmacokinetics of propranolol is altered in this patient?
Clearance increases because hepatic enzymes are induced in cirrhosis, shortening half-life
Vd decreases due to ascites, causing lower peak concentrations after oral dosing
Oral bioavailability decreases due to impaired absorption, lowering AUC at the same dose
Oral bioavailability increases due to reduced first-pass metabolism, raising AUC at the same dose
Explanation
This question tests the application of pharmacokinetics in clinical scenarios, specifically understanding how pharmacokinetic parameters affect drug dosing and efficacy. Pharmacokinetics involves absorption, distribution, metabolism, and excretion (ADME) of drugs. Key parameters include half-life, volume of distribution, and clearance. In this case, the cirrhotic patient experiences propranolol toxicity from increased systemic exposure, illustrating reduced first-pass metabolism in liver disease. The correct answer, A, is based on increased oral bioavailability raising AUC, showing why bradycardia occurs at standard doses. A common misconception is decreased bioavailability from impaired absorption, as seen in B, which fails because cirrhosis primarily affects metabolism, not absorption for this drug. To teach this, focus on first-pass effects and bioavailability changes. Encourage students to compare oral vs. IV kinetics in hepatic impairment.
A 60-year-old woman with primary biliary cholangitis and cirrhosis is started on oral morphine for severe pain. Within 24 hours she becomes increasingly sedated with shallow respirations. Exam: RR 8/min, pinpoint pupils. Labs: total bilirubin 6.0 mg/dL, albumin 2.4 g/dL, INR 2.0, creatinine 0.9 mg/dL. Morphine has significant first-pass metabolism; hepatic clearance contributes substantially to elimination, and normal half-life is ~2–3 hours. The team reviews that impaired hepatic metabolism can increase bioavailability and reduce clearance, leading to higher plasma concentrations at a given dose. Which pharmacokinetic parameter is most affected by liver disease in this patient?
Decreased hepatic clearance and reduced first-pass metabolism increasing systemic exposure
Increased clearance leading to reduced AUC and decreased sedation risk
Increased renal elimination shortening half-life and preventing accumulation
Decreased Vd causing lower peak concentrations and less respiratory depression
Explanation
This question tests the application of pharmacokinetics in clinical scenarios, specifically understanding how pharmacokinetic parameters affect drug dosing and efficacy. Pharmacokinetics involves absorption, distribution, metabolism, and excretion (ADME) of drugs. Key parameters include half-life, volume of distribution, and clearance. In this case, the patient with cirrhosis shows morphine oversedation from increased exposure, illustrating impaired hepatic clearance and reduced first-pass. The correct answer, B, is based on decreased clearance increasing systemic exposure, showing why respiratory depression ensues. A common misconception is increased clearance reducing AUC, as seen in A, which fails because cirrhosis impairs metabolism. To teach this, focus on opioid kinetics in liver disease. Encourage students to review extraction ratios and monitor for accumulation.
A 66-year-old woman with chronic kidney disease (CKD) stage 4 presents with pneumonia and is started on intravenous gentamicin. She weighs 60 kg. Labs: creatinine 2.6 mg/dL, BUN 46 mg/dL, estimated GFR 22 mL/min/1.73 m$^2$, AST 24 U/L, ALT 20 U/L. Gentamicin is hydrophilic with Vd $\approx 0.25\ \text{L/kg}$ and is eliminated almost entirely by glomerular filtration; normal half-life is ~2 hours with normal renal function. Twelve hours after a standard dose, the measured concentration remains elevated. How would renal impairment alter the elimination of this drug?
Decreased renal clearance prolongs half-life, increasing trough concentrations and accumulation risk
Decreased renal clearance shortens half-life because less drug is filtered into urine
Renal impairment increases first-pass metabolism, reducing AUC and toxicity risk
Renal impairment primarily decreases Vd, lowering peak concentrations after IV dosing
Explanation
This question tests the application of pharmacokinetics in clinical scenarios, specifically understanding how pharmacokinetic parameters affect drug dosing and efficacy. Pharmacokinetics involves absorption, distribution, metabolism, and excretion (ADME) of drugs. Key parameters include half-life, volume of distribution, and clearance. In this case, elevated gentamicin levels in CKD result from impaired excretion, illustrating reduced renal clearance. The correct answer, A, is based on decreased clearance prolonging half-life and raising troughs, showing ototoxicity risk. A common misconception is shortened half-life from reduced clearance, as seen in B, which fails because lower clearance extends half-life. To teach this, focus on GFR-drug clearance relationships. Encourage students to use nomograms for dosing.
A 39-year-old man with bipolar disorder is taking lithium. He develops dehydration from gastroenteritis and continues his usual dose. He presents with tremor, ataxia, and confusion. Labs: creatinine 1.8 mg/dL (baseline 0.9), BUN 38 mg/dL, sodium 150 mmol/L, lithium level 2.1 mmol/L (therapeutic 0.6–1.2). Lithium is not metabolized and is eliminated by renal excretion; reduced GFR decreases clearance and prolongs half-life. How would renal impairment alter the elimination of this drug?
Renal impairment decreases Vd, which directly increases clearance and prevents toxicity
Decreased renal clearance shortens half-life by reducing tubular reabsorption
Renal impairment increases first-pass metabolism, lowering lithium levels
Decreased renal clearance prolongs half-life, increasing steady-state concentration at the same dose
Explanation
This question tests the application of pharmacokinetics in clinical scenarios, specifically understanding how pharmacokinetic parameters affect drug dosing and efficacy. Pharmacokinetics involves absorption, distribution, metabolism, and excretion (ADME) of drugs. Key parameters include half-life, volume of distribution, and clearance. In this case, lithium toxicity from dehydration-induced AKI shows impaired excretion, illustrating reduced clearance in renal impairment. The correct answer, A, is based on decreased clearance prolonging half-life and raising levels, showing neurotoxicity. A common misconception is shortened half-life from reduced clearance, as seen in B, which fails because lower clearance extends half-life. To teach this, focus on hydration and monitoring in lithium use. Encourage students to correlate GFR with levels.
A 24-year-old woman with epilepsy is started on intravenous phenytoin for status epilepticus and then transitioned to oral maintenance dosing. Therapeutic drug monitoring is performed. Phenytoin has Vd $\approx 0.7\ \text{L/kg}$ and is highly protein-bound; at therapeutic levels, elimination can approach capacity-limited kinetics. A concentration-time curve after IV loading shows an initial steep decline followed by a slower decline; later, small dose increases produce large concentration increases. Which pharmacokinetic parameter is most affected as phenytoin concentrations approach metabolic saturation?
Vd increases as enzymes saturate, causing faster elimination and lower AUC
Clearance decreases as enzymes saturate, causing a disproportionate rise in steady-state concentration
Bioavailability decreases as enzymes saturate, causing higher required doses
Half-life becomes shorter as concentration increases because elimination becomes first-order
Explanation
This question tests the application of pharmacokinetics in clinical scenarios, specifically understanding how pharmacokinetic parameters affect drug dosing and efficacy. Pharmacokinetics involves absorption, distribution, metabolism, and excretion (ADME) of drugs. Key parameters include half-life, volume of distribution, and clearance. In this case, phenytoin kinetics show saturation effects, illustrating capacity-limited elimination. The correct answer, A, is based on decreased clearance at saturation causing nonlinear rises, showing monitoring importance. A common misconception is increased Vd speeding elimination, as seen in B, which fails because saturation affects clearance, not Vd. To teach this, focus on Michaelis-Menten kinetics. Encourage students to plot concentration-dose relationships.
A 59-year-old man with atrial fibrillation is started on intravenous amiodarone. He later transitions to oral dosing. Amiodarone is highly lipophilic with very large Vd ($>50\ \text{L/kg}$) and a very long terminal half-life (weeks) due to extensive tissue distribution and slow release. After stopping therapy because of bradycardia, the patient continues to have drug effects for several weeks. Which pharmacokinetic parameter best explains the prolonged persistence of drug effect after discontinuation?
Very large Vd with slow redistribution prolongs terminal half-life and delays elimination
Low protein binding increases renal filtration and rapidly clears the drug
First-pass metabolism increases AUC and shortens time to steady state
High oral bioavailability causes faster clearance and shorter terminal half-life
Explanation
This question tests the application of pharmacokinetics in clinical scenarios, specifically understanding how pharmacokinetic parameters affect drug dosing and efficacy. Pharmacokinetics involves absorption, distribution, metabolism, and excretion (ADME) of drugs. Key parameters include half-life, volume of distribution, and clearance. In this case, prolonged amiodarone effects post-discontinuation arise from tissue storage, illustrating large Vd and long half-life. The correct answer, A, is based on large Vd prolonging terminal half-life, showing persistence reasons. A common misconception is high bioavailability causing faster clearance, as seen in B, which fails because bioavailability affects exposure, not elimination duration. To teach this, focus on multi-compartment models. Encourage students to consider redistribution in lipophilic drugs.
A 33-year-old woman receives a single IV bolus of a new antibiotic. Plasma concentrations are measured: 8 mg/L at 1 hour, 4 mg/L at 5 hours, and 2 mg/L at 9 hours. The drug follows first-order elimination with one-compartment kinetics and Vd $\approx 20\ \text{L}$. No renal or hepatic disease is present. Based on these data, which pharmacokinetic parameter is most directly estimated from the slope of the log concentration vs time relationship?
Volume of distribution (Vd), which equals dose divided by AUC
Maximum effect (Emax), which determines potency from dose-response curves
Elimination rate constant ($k$), which determines half-life via $t_{1/2}=0.693/k$
Bioavailability ($F$), which determines AUC after IV dosing
Explanation
This question tests the application of pharmacokinetics in clinical scenarios, specifically understanding how pharmacokinetic parameters affect drug dosing and efficacy. Pharmacokinetics involves absorption, distribution, metabolism, and excretion (ADME) of drugs. Key parameters include half-life, volume of distribution, and clearance. In this case, antibiotic concentration decline follows first-order kinetics, illustrating exponential elimination. The correct answer, A, is based on k determining half-life from log-plot slope, showing direct estimation. A common misconception is bioavailability determining IV AUC, as seen in B, which fails because F=1 for IV. To teach this, focus on one-compartment models. Encourage students to calculate k from data points.
A 57-year-old woman with rheumatoid arthritis is stable on oral methotrexate. She begins high-dose ibuprofen for pain. One week later, she develops mouth ulcers and fatigue. Labs: creatinine 1.6 mg/dL (baseline 0.9), BUN 32 mg/dL, AST 24 U/L, ALT 20 U/L, WBC 2.9 $\times 10^3$/mcL. Methotrexate is primarily renally eliminated and is secreted by renal tubules; NSAIDs can reduce renal perfusion and compete for tubular secretion, decreasing methotrexate clearance. Which of the following best describes how the pharmacokinetics of methotrexate is altered in this patient?
Increased renal clearance decreases AUC, causing therapeutic failure
Increased Vd increases clearance and prevents accumulation during chronic dosing
Decreased absorption increases AUC by reducing first-pass metabolism
Decreased renal clearance increases AUC and prolongs half-life, raising toxicity risk
Explanation
This question tests the application of pharmacokinetics in clinical scenarios, specifically understanding how pharmacokinetic parameters affect drug dosing and efficacy. Pharmacokinetics involves absorption, distribution, metabolism, and excretion (ADME) of drugs. Key parameters include half-life, volume of distribution, and clearance. In this case, methotrexate toxicity from ibuprofen arises from renal interactions, illustrating decreased clearance causing accumulation. The correct answer, A, is based on reduced renal clearance increasing AUC and half-life, showing myelosuppression risk. A common misconception is increased clearance decreasing AUC, as seen in B, which fails because NSAIDs impair renal function. To teach this, focus on tubular secretion competitions. Encourage students to monitor for interactions.
A 68-year-old man with cirrhosis is started on a continuous IV infusion of a hepatically metabolized sedative. The drug has Vd 40 L and normal clearance 40 L/h (half-life $\approx 0.693\times Vd/CL \approx 0.7$ h). In this patient, clearance is estimated to be 10 L/h due to hepatic dysfunction, with Vd unchanged. No renal impairment is present. What adjustment to the dosing regimen is most appropriate given the pharmacokinetic data?
Keep infusion rate unchanged because Vd, not clearance, determines steady-state concentration
Decrease the infusion rate to match reduced clearance and prevent a higher steady-state concentration
Increase the infusion rate because reduced clearance lowers drug exposure
Increase dosing frequency because longer half-life shortens time to steady state
Explanation
This question tests the application of pharmacokinetics in clinical scenarios, specifically understanding how pharmacokinetic parameters affect drug dosing and efficacy. Pharmacokinetics involves absorption, distribution, metabolism, and excretion (ADME) of drugs. Key parameters include half-life, volume of distribution, and clearance. In this case, sedative infusion in cirrhosis requires adjustment for lower clearance, illustrating steady-state dependence on infusion rate and clearance. The correct answer, A, is based on decreasing rate to maintain target concentration, showing prevention of excess. A common misconception is increasing rate for reduced clearance, as seen in B, which fails because lower clearance raises steady-state levels. To teach this, focus on Css = rate/CL equation. Encourage students to calculate adjustments using kinetics.
A 45-year-old man with CKD stage 4 is prescribed a renally eliminated antiviral. The drug follows first-order kinetics and has Vd 30 L. In healthy adults, clearance is 6 L/h (half-life $\approx 3.5$ h). In this patient, measured clearance is 2 L/h due to reduced GFR. No hepatic disease. Which of the following best describes how the pharmacokinetics of this drug is altered in this patient?
Bioavailability increases because renal impairment reduces first-pass hepatic metabolism
Half-life decreases because reduced clearance lowers plasma concentration more quickly
Half-life increases because clearance decreases while Vd remains constant
Vd decreases and therefore AUC decreases despite reduced clearance
Explanation
This question tests the application of pharmacokinetics in clinical scenarios, specifically understanding how pharmacokinetic parameters affect drug dosing and efficacy. Pharmacokinetics involves absorption, distribution, metabolism, and excretion (ADME) of drugs. Key parameters include half-life, volume of distribution, and clearance. In this case, a patient with chronic kidney disease receiving a renally eliminated antiviral illustrates the impact of decreased clearance on half-life. The correct answer, Choice A, is based on the half-life formula (t1/2 = 0.693 * Vd / CL), showing that decreased clearance with constant Vd leads to increased half-life. A common misconception is that reduced clearance accelerates drug elimination, as seen in Choice B, which fails because reduced clearance actually prolongs the drug's presence in the plasma. To teach this, focus on renal elimination pathways and how they are impaired in kidney disease. Encourage students to calculate half-life using given parameters and apply concepts to dosing adjustments in renal impairment.