DHA Pharmacist Pharmacokinetics and Pharmacodynamics 1 — Questions and Answers
Question 1: A drug has a volume of distribution (Vd) of 700 L in a 70 kg patient. What does this suggest?
- The drug is largely confined to plasma
- The drug is extensively distributed into tissues (Correct answer)
- The drug has poor oral bioavailability
- The drug undergoes significant first-pass metabolism
Correct answer: The drug is extensively distributed into tissues
A large Vd (much greater than total body water ~42 L) indicates the drug distributes extensively into peripheral tissues rather than remaining in plasma.
Question 2: A drug has a half-life of 8 hours. Approximately how long will it take to reach steady-state plasma concentration with regular dosing?
- 8 hours
- 16 hours
- 40 hours (Correct answer)
- 80 hours
Correct answer: 40 hours
Steady-state is reached after approximately 4–5 half-lives; 5 × 8 hours = 40 hours.
Question 3: Which route of administration completely bypasses first-pass hepatic metabolism?
- Oral
- Rectal (proximal)
- Intravenous (Correct answer)
- Sublingual and intravenous only
Correct answer: Intravenous
Intravenous administration delivers the drug directly into systemic circulation, completely bypassing hepatic first-pass metabolism.
Question 4: The bioavailability of an oral drug is 25%. If the IV dose producing the desired effect is 20 mg, what oral dose is required to achieve the same effect?
- 5 mg
- 20 mg
- 40 mg
- 80 mg (Correct answer)
Correct answer: 80 mg
Oral dose = IV dose / bioavailability = 20 mg / 0.25 = 80 mg to achieve equivalent systemic exposure.
Question 5: Hepatic clearance of a high-extraction drug (extraction ratio > 0.7) is primarily dependent on:
- Plasma protein binding
- Hepatic blood flow (Correct answer)
- Renal function
- Glomerular filtration rate
Correct answer: Hepatic blood flow
High-extraction drugs are efficiently cleared by the liver on first pass, so their clearance is limited by hepatic blood flow, not enzyme capacity.
Question 6: A drug follows first-order kinetics. Which of the following statements is TRUE?
- A constant amount of drug is eliminated per unit time
- The rate of elimination is proportional to drug concentration (Correct answer)
- Elimination pathways become saturated at therapeutic doses
- Half-life increases as plasma concentration increases
Correct answer: The rate of elimination is proportional to drug concentration
In first-order kinetics, a constant fraction (not amount) of drug is eliminated per unit time, meaning the rate is proportional to concentration.
Question 7: Which pharmacokinetic parameter is most useful when calculating a maintenance dose to maintain a target steady-state plasma concentration?
- Volume of distribution (Vd)
- Half-life (t½)
- Total body clearance (CL) (Correct answer)
- Peak plasma concentration (Cmax)
Correct answer: Total body clearance (CL)
Maintenance dose rate = CL × target Css; clearance determines how much drug must be given per unit time to replace what is eliminated.
DHA Pharmacist Pharmacokinetics and Pharmacodynamics 1
A drug has a volume of distribution (Vd) of 700 L in a 70 kg patient.
What does this suggest?