Pharmacokinetics

Which of the following best describes the relationship between plasma protein binding and drug distribution volume (Vd)?

Which of the following drugs exhibits zero-order kinetics at therapeutic concentrations?

A 62-year-old man with chronic kidney disease (eGFR 25 mL/min/1.73m²) is prescribed gentamicin 80 mg IV for a urinary tract infection. His serum creatinine is 3.2 mg/dL. On day 3 of therapy, his serum gentamicin peak level is 8 µg/mL (normal peak 4–10 µg/mL) but trough is 2.5 µg/mL (normal trough <2 µg/mL). Which pharmacokinetic parameter is primarily altered in this patient, leading to drug accumulation?

A drug undergoes first-order elimination with a half-life of 6 hours. What percentage of the drug will be eliminated after 18 hours?

A 38-year-old woman on warfarin for atrial fibrillation is started on rifampicin for tuberculosis. After 2 weeks, her INR drops from 2.8 to 1.5 despite unchanged warfarin dose. Which investigation is most appropriate to confirm the suspected drug–drug interaction and guide therapeutic adjustment?