Medical Pharmacology Question Bank

Chapter 2: Pharmacokinetics — Module 3: Metabolism and Excretion
Tier: Tier 2 — Conceptual Understanding

1. Drug clearance is best defined as which of the following?

A) The time required for plasma drug concentration to fall by 50%
B) The volume of plasma completely cleared of drug per unit time, expressed in
C) The total amount of drug eliminated from the body per day, expressed in milligrams
D) The fraction of an oral dose that survives first-pass hepatic metabolism
E) The rate at which drug moves from the plasma compartment into peripheral tissues

ANSWER: B

Rationale:

Clearance is the pharmacokinetic parameter that quantifies the body's efficiency in eliminating a drug. It is defined as the volume of plasma from which drug is completely removed per unit time — it does not mean that a defined volume of plasma is literally cleaned out, but rather that the rate of drug elimination equals what would result from completely clearing that volume of drug-containing plasma per unit time. For a drug eliminated by both kidney and liver, total clearance = renal clearance + hepatic clearance + other clearance pathways. Clearance is related to half-life by: t½ = 0.693 × Vd / CL — a larger clearance produces a shorter half-life, and a larger volume of distribution produces a longer half-life.

2. A drug has a half-life of 12 hours. A patient starts taking it at a fixed dose once every 12 hours. Approximately how long will it take for plasma drug concentrations to reach steady state?

A) 12 hours — one half-life after the first dose
B) 24 hours — after two doses have been administered
C) 48-60 hours — approximately 4-5 half-lives after initiation
D) 120 hours — exactly 10 half-lives, when 99.9% of maximum accumulation is reached
E) The time to steady state depends on the size of the dose, not on the half-life

ANSWER: C

Rationale:

A fundamental pharmacokinetic principle is that the time to reach steady state depends exclusively on the drug's elimination half-life and is entirely independent of the dose size or the dosing interval. Plasma concentrations approach steady state according to a predictable pattern: approximately 50% after 1 half-life, 75% after 2 half-lives, 87.5% after 3 half-lives, 93.75% after 4 half-lives, and approximately 97% after 5 half-lives — the conventional criterion for considering steady state achieved. For a drug with a 12-hour half-life: 4-5 × 12 hours = 48-60 hours. This principle has a critical corollary: the time to reach a new steady state after any dose change (increase, decrease, or discontinuation) is also 4-5 half-lives. Option E contains the most important misconception to correct: doubling the dose does NOT halve the time to reach steady state — it only doubles the concentration at steady state while leaving the time unchanged.

3. Which of the following correctly describes first-order drug elimination kinetics?

A) A constant amount of drug is eliminated per unit time, regardless of the plasma
B) A constant fraction of the drug present in the body is eliminated per unit time;
C) Drug elimination rate increases exponentially with increasing plasma concentration
D) Two different enzymes eliminate the drug simultaneously at different rates — first-
E) The drug is eliminated by a single renal mechanism (glomerular filtration) without

ANSWER: B

Rationale:

First-order kinetics — also called linear kinetics — is the most common pattern of drug elimination. Its defining characteristic is that the rate of elimination is directly proportional to the drug concentration: as concentration doubles, the rate of elimination doubles. This produces several important consequences: (1) a constant fraction of drug is eliminated per unit time (rather than a constant amount); (2) the drug has a fixed, concentration-independent half-life; (3) doubling the dose doubles the steady-state plasma concentration; and (4) plasma concentration declines in an exponential (curved on linear scale, straight line on semi-logarithmic scale) pattern over time.

4. A drug is given as an intravenous bolus. The following plasma concentrations are measured: at 0 hours: 80 mg/L; at 6 hours: 40 mg/L; at 12 hours: 20 mg/L; at 18 hours: 10 mg/L. Which statement is correct?

A) The drug follows zero-order kinetics because the concentration decreases by
B) The drug follows first-order kinetics with a half-life of 6 hours — the
C) The drug follows first-order kinetics with a half-life of 12 hours — it takes
D) The data are insufficient to determine the elimination kinetics — Cmax, Tmax,
E) The drug follows zero-order kinetics because the absolute amount eliminated

ANSWER: B

Rationale:

The diagnostic test for first-order kinetics is whether plasma concentration decreases by a constant fraction (not a constant amount) per unit time. Examining the data: 80 → 40 (50% decrease in 6 hours), 40 → 20 (50% decrease in 6 hours), 20 → 10 (50% decrease in 6 hours). The concentration decreases by exactly half every 6 hours — the defining feature of first-order kinetics. The half-life is 6 hours. The absolute amount eliminated does decrease over time (40 → 20 → 10 mg/L per interval), but this is consistent with first-order kinetics because a fixed fraction of a decreasing amount produces a decreasing absolute amount. Options A and E make the common error of confusing decreasing absolute elimination rate with zero-order kinetics. Zero-order kinetics would show a constant absolute decrease per time interval (e.g., 80 → 40 → 0 at -40 mg/L per 6 hours).

5. Phenytoin is unusual among common antiepileptic drugs because it exhibits capacity-limited (Michaelis-Menten) elimination at therapeutic plasma concentrations. Which of the following best describes the clinical consequence of this kinetic behavior?

A) Phenytoin has a very short half-life at therapeutic concentrations because its
B) Phenytoin doses can be increased in large increments (e.g., 50-100 mg at a time)
C) Small increases in phenytoin dose can produce disproportionately large increases
D) Phenytoin's plasma concentration is unaffected by changes in dose because the
E) Phenytoin's elimination kinetics are identical to most other drugs — the

ANSWER: C

Rationale:

The Michaelis-Menten (capacity-limited) kinetics of phenytoin at therapeutic plasma concentrations is one of the most clinically important kinetic properties in all of pharmacology. At therapeutic phenytoin concentrations (10-20 mg/L), the CYP2C9 enzyme responsible for phenytoin's metabolism is operating near its maximum capacity (Vmax) — the enzyme is nearly saturated with substrate. When a drug's elimination system is saturated: (1) a small increase in dose produces a disproportionately large increase in steady-state plasma concentration; (2) the drug lacks a fixed half-life (half-life increases as concentration rises, making time to steady state unpredictable); (3) there is a maximum sustainable daily dose beyond which accumulation is unlimited. These properties make phenytoin dose adjustment uniquely dangerous — increments of more than 25-50 mg at a time can convert a therapeutic level to a toxic one. This is why phenytoin is managed with therapeutic drug monitoring and very small dose adjustments.

6. The three processes by which the kidney handles drug excretion are glomerular filtration, tubular secretion, and tubular reabsorption. For a drug with the following properties — fu (free fraction) = 0.40, GFR = 120 mL/min, and measured renal clearance = 250 mL/min — which renal handling mechanism must be operating?

A) Glomerular filtration alone, because the drug's molecular size allows complete
B) Net tubular reabsorption, because the measured renal clearance (250 mL/min) is
C) Active tubular secretion must be contributing, because the measured renal
D) The data are internally inconsistent — renal clearance cannot exceed GFR
E) Carrier-mediated tubular reabsorption, because the drug is 60% protein-bound

ANSWER: C

Rationale:

This question demonstrates how comparing measured renal clearance to calculated filtration clearance reveals the mechanisms of renal drug handling. Glomerular filtration is passive and non-saturable — only the free (unbound) fraction of drug is filtered. Maximum filtration clearance = fu × GFR = 0.40 × 120 mL/min = 48 mL/min. If the measured renal clearance (250 mL/min) is substantially greater than the filtration clearance (48 mL/min), the difference must be accounted for by active tubular secretion: secretion clearance ≈ 250 - 48 = 202 mL/min. Active tubular secretion can clear even protein-bound drug — the high-affinity organic anion (OAT1, OAT3) or organic cation (OCT2, MATE) transporters pump drug from peritubular capillary blood into the tubular lumen, and as free drug is removed, bound drug rapidly dissociates from albumin to replace it.

7. A 78-year-old woman has a serum creatinine of 0.7 mg/dL — within the normal laboratory reference range. Her physician plans to use standard dosing for an aminoglycoside antibiotic. A clinical pharmacist raises a concern about potential toxicity. Which pharmacokinetic principle supports the pharmacist's concern?

A) Elderly women metabolize aminoglycosides more rapidly via CYP1A2, requiring
B) A normal serum creatinine in an elderly, low-body-weight patient may significantly
C) Aminoglycoside toxicity in elderly patients is exclusively pharmacodynamic —
D) A serum creatinine of 0.7 mg/dL is below normal for an adult female and indicates
E) The pharmacist's concern is unfounded — serum creatinine is the gold standard

ANSWER: B

Rationale:

Serum creatinine is an imperfect marker of renal function in elderly, low-body-weight patients because creatinine production is proportional to muscle mass (creatinine is generated by non-enzymatic degradation of muscle creatine phosphate). An elderly woman with sarcopenia produces less creatinine per day than a young, muscular adult — her serum creatinine is kept low not because her kidneys are efficient, but because very little creatinine is being produced. The Cockcroft-Gault equation corrects for this by incorporating age and body weight: CrCl = [(140 - age) × weight / (72 × serum creatinine)] × 0.85 (for women). For a 78-year-old woman weighing 45 kg with serum creatinine 0.7 mg/dL: CrCl = [(140-78) × 45 / (72 × 0.7)] × 0.85 = [62 × 45 / 50.4] × 0.85 = 47 mL/min — substantially below normal (120 mL/min) despite the apparently normal serum creatinine. Since aminoglycosides are 100% renally eliminated (fe = 1.0), this reduced CrCl means the aminoglycoside half-life is prolonged, accumulation is likely with standard dosing, and toxicity risk — both nephrotoxicity and ototoxicity — is substantially elevated.

8. At steady state, which of the following equations correctly describes the average steady-state plasma concentration of a drug given orally at regular intervals?

A) Css_avg = Dose × half-life / Volume of distribution
B) Css_avg = (F × Dose / τ) / CL, where F is bioavailability, τ is the dosing
C) Css_avg = Loading dose / Volume of distribution
D) Css_avg = Dose × Clearance / Bioavailability
E) Css_avg = Volume of distribution × half-life / Dose

ANSWER: B

Rationale:

At steady state, the rate of drug administration into the systemic circulation equals the rate of drug elimination: Rate in = Rate out. Rate in = (F × Dose) / τ (the amount of drug reaching systemic circulation per dosing interval). Rate out = CL × Css_avg. Setting them equal: (F × Dose) / τ = CL × Css_avg. Solving for Css_avg: Css_avg = (F × Dose / τ) / CL. This equation reveals several important clinical principles: (1) steady-state concentration is directly proportional to dose and bioavailability — doubling the dose doubles Css_avg; (2) Css_avg is inversely proportional to clearance — if clearance falls by 50% (e.g., renal impairment for a renally cleared drug), Css_avg doubles at the same dose; (3) the dosing interval τ appears in the denominator — a shorter dosing interval increases Css_avg, and a longer interval decreases it (for the same total daily dose, divided doses produce higher Css_avg than less frequent dosing). Note that half-life and Vd do not directly appear in the Css_avg equation — they determine the time to reach steady state, not the level at steady state.

9. A patient with normal renal function takes drug X at a fixed dose twice daily. His physician reduces total daily dose by 50% for an unrelated reason. How does this dose reduction affect: (1) the steady-state plasma concentration, and (2) the time required to reach the new steady state?

A) Css_avg decreases by 50%; time to new steady state is halved because the lower
B) Css_avg decreases by 50%; time to reach the new (lower) steady state remains
C) Css_avg decreases by 75% because dose reduction always produces a greater
D) Css_avg is unchanged because the body's clearance mechanism compensates for
E) Time to reach the new steady state doubles when dose is halved, because less

ANSWER: B

Rationale:

This question tests two separate and independent pharmacokinetic principles. First, Css_avg is directly proportional to dose (from the equation Css_avg = F × Dose / (CL × τ)) — halving the dose halves Css_avg, assuming the drug follows first-order linear kinetics where clearance is dose-independent. Second, the time to reach any steady state — whether initial steady state or a new steady state after a dose change — is determined exclusively by the drug's elimination half-life and equals approximately 4-5 half-lives. Halving the dose does not change the drug's half-life (which depends on Vd and CL, not dose). Therefore the time course of approach to the new steady state is identical to the original — still 4-5 half-lives. This has an important clinical corollary: when a clinician changes a patient's dose, they should wait 4-5 half-lives before drawing a drug level to confirm the new steady state has been achieved. Drawing a level too early will underestimate the eventual steady-state concentration for dose increases, or overestimate it for dose decreases.

10. A drug with a half-life of 8 hours is given as an intravenous bolus of 200 mg. Using the principle of first-order elimination, approximately what plasma concentration would be expected 24 hours after the dose if the initial concentration (C0) was 10 mg/L?

A) 5.0 mg/L — the concentration falls by half every 24 hours (one half-life is 24 hours)
B) 1.25 mg/L — at 24 hours, three half-lives have elapsed (24/8 = 3), so the
C) 0.625 mg/L — at 24 hours, four half-lives have elapsed
D) 2.5 mg/L — at 24 hours, two half-lives have elapsed (24/8 = 2), so the
E) 0 mg/L — after more than two half-lives, essentially all drug has been eliminated

ANSWER: B

Rationale:

For first-order elimination, plasma concentration at any time t after a dose is given by: C(t) = C0 × (0.5)^(t/t½) = C0 × e^(-k×t), where k = 0.693/t½. At t = 24 hours with t½ = 8 hours: number of half-lives = 24/8 = 3 half-lives. C(24h) = 10 mg/L × (0.5)^3 = 10 × 0.125 = 1.25 mg/L. Checking against the half-life approach: after 1 half-life (8 hours): 10 → 5 mg/L; after 2 half-lives (16 hours): 5 → 2.5 mg/L; after 3 half-lives (24 hours): 2.5 → 1.25 mg/L. Option D makes the error of assuming only two half-lives have elapsed in 24 hours — this would be correct if the half-life were 12 hours.