Pharmacokinetics: Absorption and Distribution — MCQs
A patient given digoxin started having side effects like nausea and vomiting. The serum concentration of digoxin was 4 ng/mL. The plasma therapeutic range is 1-2 ng/mL. If the half-life of digoxin is 40 hours, how long should one wait before resuming the treatment?
Drug X has an affinity for albumin, while drug Y has 150 times greater affinity. Which of the following statements is MOST accurate?
Which of the following drugs is known to have low first pass metabolism?
Which of the following describes the first-pass metabolism?
Diffusion of lipid-insoluble substances across the cell membrane depends on all of the following factors except which one?
Brand A of liposomal amphotericin B is of innovator company and brand B is of a generic company. AUC of Brand A is 124 mg.h/L and AUC of brand B is 115 mg.h/L. Which of the following statements is correct?
Which calcium channel blocker has the shortest duration of action?
Which of the following is excreted in saliva?
Low apparent volume of distribution of drug indicates that:
The following plot comparing pharmacokinetics of different ibuprofen brands is called:
Pharmacokinetics: Absorption and Distribution Indian Medical PG Practice Questions and MCQs
Question 1: A patient given digoxin started having side effects like nausea and vomiting. The serum concentration of digoxin was 4 ng/mL. The plasma therapeutic range is 1-2 ng/mL. If the half-life of digoxin is 40 hours, how long should one wait before resuming the treatment?
- A. 120 hours
- B. 140-180 hours
- C. 1 half-life (40 hours)
- D. 80 hours (Correct Answer)
Explanation: ***80 hours (2 half-lives)***- Current digoxin level is **4 ng/mL**, which is **twice the upper therapeutic limit** (2 ng/mL), causing toxicity with nausea and vomiting [1]- After **1 half-life (40 hours)**: concentration reduces to 2 ng/mL (upper therapeutic limit) [2]- After **2 half-lives (80 hours)**: concentration reduces to 1 ng/mL (mid-therapeutic range) [2]- **Clinical rationale**: While 2 ng/mL is technically within range, waiting for 2 half-lives ensures the level is comfortably in the **middle of the therapeutic window** (1 ng/mL), providing a **safer margin** before resuming treatment in a patient who just experienced toxicity- This conservative approach minimizes risk of recurrent toxicity, especially important given the patient's recent symptoms at 4 ng/mL*1 half-life (40 hours)*- After 1 half-life, digoxin level would be 2 ng/mL, which is at the **upper limit** of the therapeutic range- While technically within the therapeutic range, this leaves **minimal safety margin** in a patient who just experienced toxicity- Starting treatment immediately at this level carries higher risk of recurrent side effects*120 hours (3 half-lives)*- After 3 half-lives, the concentration would be **0.5 ng/mL**, which is **below the therapeutic range** (1-2 ng/mL)- This is overly conservative and would **unnecessarily delay** resumption of essential cardiac medication- Could lead to inadequate control of the underlying condition (heart failure or atrial fibrillation)*140-180 hours (3.5-4.5 half-lives)*- This would reduce digoxin to **0.25-0.35 ng/mL**, well below therapeutic levels- This **excessive delay** is not clinically justified and could worsen the patient's cardiac condition- No standard protocol recommends waiting this long before resuming digoxin therapy
Question 2: Drug X has an affinity for albumin, while drug Y has 150 times greater affinity. Which of the following statements is MOST accurate?
- A. Drug X will be more available in tissues
- B. Drug Y will be less available in tissues
- C. Toxicity of drug Y may be influenced by multiple factors, not just its binding.
- D. The free concentration of drug X in blood is higher, facilitating tissue distribution. (Correct Answer)
Explanation: ***Correct: The free concentration of drug X in blood is higher, facilitating tissue distribution.*** - This is the **MOST accurate and complete** answer because it directly addresses the pharmacokinetic mechanism - Drug X has **lower affinity for albumin** → larger proportion remains **unbound (free)** in plasma - Only **free (unbound) drug** can cross capillary membranes to distribute into tissues - This statement precisely explains both the **cause** (higher free concentration) and **effect** (facilitating tissue distribution) *Drug X will be more available in tissues* - This statement is **factually true** and follows logically from drug X's lower protein binding - However, it's **less precise** than the correct answer because it doesn't explicitly explain the **mechanism** (higher free concentration) - The term "available" is less specific than "free concentration," which is the key pharmacokinetic parameter *Drug Y will be less available in tissues* - This statement is also **factually true** - drug Y's **150× higher albumin affinity** means more drug is bound - Higher protein binding → **smaller free fraction** → less tissue distribution - However, like option 1, this doesn't explicitly state the **mechanistic principle** involving free drug concentration - The question asks for the MOST accurate statement, and this focuses on drug Y rather than explaining the core concept *Toxicity of drug Y may be influenced by multiple factors, not just its binding* - While this is a **true general principle**, it's **not directly relevant** to the specific question - This statement doesn't address the **pharmacokinetic implications** of differential albumin binding - It's too vague and doesn't demonstrate understanding of the relationship between protein binding and tissue distribution - The question specifically asks about the affinity differences and their consequences
Question 3: Which of the following drugs is known to have low first pass metabolism?
- A. Lidocaine
- B. Propranolol
- C. Theophylline (Correct Answer)
- D. Morphine
Explanation: ***Theophylline*** - **Theophylline** exhibits **low first-pass metabolism**, meaning a significant portion of the orally administered drug reaches systemic circulation unchanged. - This characteristic contributes to its relatively **high bioavailability** when given orally. *Lidocaine* - **Lidocaine** undergoes extensive **first-pass metabolism** in the liver, leading to very low oral bioavailability. - Due to this, it is typically administered **parenterally** (e.g., intravenously or topically) to achieve therapeutic concentrations. *Propranolol* - **Propranolol** is known for its significant **first-pass metabolism**, which results in a much lower bioavailability after oral administration compared to intravenous. - This extensive metabolism necessitates higher oral doses to achieve the same therapeutic effect as parenteral administration. *Morphine* - **Morphine** also undergoes substantial **first-pass metabolism** in the liver, where it is primarily glucuronidated. - This leads to a lower oral bioavailability compared to other routes of administration and contributes to a higher oral dose requirement.
Question 4: Which of the following describes the first-pass metabolism?
- A. Drug given orally is metabolized by the liver before entering the circulation. (Correct Answer)
- B. Drug given intravenously bypasses the liver initially.
- C. Gastric acids primarily affect the stability of drugs.
- D. Absorption of a drug occurs in the intestines.
Explanation: ***Drug given orally is metabolized by the liver before entering the circulation.*** - **First-pass metabolism**, also known as **presystemic metabolism**, refers to the phenomenon where a drug is extensively metabolized in the **gastrointestinal tract** and **liver** before it reaches systemic circulation. - This process significantly reduces the **bioavailability** of orally administered drugs, as a substantial portion of the drug is inactivated or converted to metabolites before it can exert its therapeutic effect. *Drug given intravenously bypasses the liver initially.* - While intravenous (IV) administration does bypass **first-pass metabolism** in the liver and gastrointestinal tract, this statement describes what happens with IV drugs, not the first-pass metabolism itself. - IV drugs enter the **systemic circulation** directly, achieving 100% bioavailability, unlike orally administered drugs affected by first-pass metabolism. *Gastric acids primarily affect the stability of drugs.* - **Gastric acids** primarily affect the **chemical stability** and degradation of certain drugs, but this is a separate phenomenon from first-pass metabolism. - While acid degradation can reduce drug absorption, first-pass effect specifically refers to metabolic transformation in the gut wall and liver. *Absorption of a drug occurs in the intestines.* - The **small intestine** is indeed the primary site for drug absorption due to its large surface area and rich blood supply. - However, this statement describes **drug absorption** in general, not specifically the process of first-pass metabolism, which occurs *after* absorption and involves metabolism before systemic circulation.
Question 5: Diffusion of lipid-insoluble substances across the cell membrane depends on all of the following factors except which one?
- A. Hydrated radius
- B. Electrical charge
- C. Lipid solubility (Correct Answer)
- D. Shape
Explanation: ***Lipid solubility*** - This property is crucial for substances that **readily diffuse directly through the lipid bilayer**. - Lipid-insoluble substances, by definition, **cannot diffuse through the lipid bilayer based on their lipid solubility**, requiring other mechanisms or factors like channels or carriers. *Hydrated radius* - The **size of a hydrated ion or molecule** is a critical determinant for its ability to pass through specific protein channels or pores in the cell membrane. - A larger hydrated radius impedes passage through narrow channels, directly affecting the diffusion of lipid-insoluble substances. *Electrical charge* - For **charged lipid-insoluble substances** (ions), their movement across the membrane is significantly influenced by the **transmembrane electrical potential difference**. - The electrical gradient can either facilitate or hinder the diffusion of these substances through channels or transporters. *Shape* - The **three-dimensional configuration** of a lipid-insoluble substance can affect its ability to bind to and pass through specific protein channels or carrier proteins. - A substance's shape must complement the architecture of the transport mechanism for efficient diffusion.
Question 6: Brand A of liposomal amphotericin B is of innovator company and brand B is of a generic company. AUC of Brand A is 124 mg.h/L and AUC of brand B is 115 mg.h/L. Which of the following statements is correct?
- A. Brand A is bioequivalent to brand B (Correct Answer)
- B. Brand A is not bioequivalent to brand B
- C. Brand A has higher volume of distribution than brand B
- D. Brand B has higher volume of distribution than brand A
Explanation: ***Brand A is bioequivalent to brand B*** - **Bioequivalence** is generally established if the **90% confidence interval** for the ratio of the **AUC** (and Cmax) of the test product (Brand B) to the reference product (Brand A) falls within **80-125%**. - Here, the ratio of AUC (Brand B / Brand A) is 115/124 ≈ 0.927 or 92.7%. This value falls well within the accepted range of 80-125%, indicating bioequivalence. *Brand A is not bioequivalent to brand B* - This statement is incorrect because the AUC ratio (115/124 ≈ 0.927) is within the **standard bioequivalence range of 80-125%**, indicating that the two brands are indeed bioequivalent. - While there are differences in Cmax and Tmax for the two brands as shown in the graph, the provided AUC values suggest bioequivalence for the overall drug exposure. *Brand A has higher volume of distribution than brand B* - The provided **AUC values** (Area Under the Curve) primarily reflect the **extent of drug exposure** and are not directly indicative of the **volume of distribution (Vd)**. - Vd is a pharmacokinetic parameter that relates the total amount of drug in the body to the concentration of the drug in plasma, and it cannot be directly inferred from AUC values alone without additional information like dose and clearance. *Brand B has higher volume of distribution than brand A* - Similar to the previous option, AUC values alone are insufficient to determine the relative **volume of distribution** between the two brands. - Changes in Vd would affect the peak concentration (Cmax) and the elimination half-life, but a definitive conclusion requires more comprehensive pharmacokinetic analysis.
Question 7: Which calcium channel blocker has the shortest duration of action?
- A. Diltiazem
- B. Amlodipine
- C. Nimodipine (Correct Answer)
- D. Verapamil
Explanation: ***Nimodipine*** - Nimodipine is a **dihydropyridine calcium channel blocker** specifically formulated for cerebral vasodilation and used in conditions like **subarachnoid hemorrhage**. - It has a relatively **short half-life** and rapid onset, making its duration of action shorter compared to other commonly used calcium channel blockers. *Amlodipine* - Amlodipine is known for its **long duration of action** and once-daily dosing due to its slow absorption and high bioavailability. - Its prolonged action is beneficial for conditions like **hypertension and angina**, where sustained vasodilation is desired. *Diltiazem* - Diltiazem's duration of action is **intermediate** compared to other calcium channel blockers, often requiring BID to TID dosing for immediate-release formulations. - It's a **non-dihydropyridine calcium channel blocker** with effects on both vascular smooth muscle and cardiac conduction. *Verapamil* - Verapamil also has an **intermediate duration of action**, similar to diltiazem, with immediate-release forms requiring multiple daily doses. - As a **non-dihydropyridine calcium channel blocker**, it has significant effects on myocardial contractility and AV nodal conduction.
Question 8: Which of the following is excreted in saliva?
- A. Lithium (Correct Answer)
- B. Chloramphenicol
- C. Ampicillin
- D. Tetracycline
Explanation: ***Lithium*** - **Lithium** is actively excreted in saliva, making salivary lithium levels a potential, though not routinely used, indicator of serum levels. - The salivary glands can concentrate lithium, leading to concentrations in saliva that are typically **2 to 3 times higher** than in plasma (saliva/plasma ratio of approximately 2-3:1). - This property makes saliva a useful non-invasive medium for therapeutic drug monitoring of lithium. *Chloramphenicol* - **Chloramphenicol** is primarily metabolized in the liver by glucuronidation and excreted in the urine. - While small amounts may be found in various body fluids, it is not a significant component of salivary excretion. *Ampicillin* - **Ampicillin**, a penicillin antibiotic, is mainly eliminated unchanged via renal excretion. - Salivary excretion is not a primary route of elimination for ampicillin. *Tetracycline* - **Tetracycline** antibiotics are primarily excreted unchanged by the kidneys and, to a lesser extent, in bile. - While some drugs can be detected in saliva, tetracycline is not notably excreted through this route in clinically significant amounts.
Question 9: Low apparent volume of distribution of drug indicates that:
- A. Drug has low bioavailability
- B. Drug has low efficacy
- C. Drug is not extensively distributed to tissue (Correct Answer)
- D. Drug has low half life
Explanation: ***Drug is not extensively distributed to tissue*** - A **low apparent volume of distribution (Vd)** suggests that the drug primarily remains in the **vascular compartment**. - This indicates **minimal binding to peripheral tissues** and less distribution into extravascular spaces. *Drug has low bioavailability* - **Bioavailability** refers to the fraction of an administered drug that reaches the **systemic circulation unchanged**. - While related to drug disposition, a low Vd does not directly imply low bioavailability; a drug can have high bioavailability but remain largely in the blood. *Drug has low efficacy* - **Efficacy** is the maximum effect a drug can produce regardless of the dose. - Vd relates to drug distribution, not its pharmacological effect or **intrinsic activity** at its target. *Drug has low half life* - The **half-life** of a drug is determined by its **volume of distribution (Vd)** and **clearance (CL)** (t½ = 0.693 × Vd / CL). - While a low Vd can contribute to a shorter half-life if clearance is high, Vd alone does not solely determine half-life; clearance also plays a significant role.
Question 10: The following plot comparing pharmacokinetics of different ibuprofen brands is called:
- A. Kaplan Meier plot
- B. Spaghetti plot (Correct Answer)
- C. Funnel plot
- D. Forest plot
Explanation: ***Spaghetti plot*** - A **spaghetti plot** is characterized by multiple lines, each representing an individual's data across different time points or conditions, creating a visual resemblance to strands of spaghetti. This plot is ideal for visualizing **individual changes** and patterns in longitudinal data. - In pharmacokinetics, spaghetti plots are useful to compare the **drug concentration profiles** of different brands or formulations within individuals or across a group, showing individual variability. *Kaplan Meier plot* - A Kaplan-Meier plot (or survival curve) is used to estimate the **survival probability** over time for a group of individuals. - It displays a step-wise curve that decreases over time, representing the proportion of subjects **surviving** or remaining event-free, which is clearly not what is depicted in the image. *Funnel plot* - A **funnel plot** is a scatter plot used in meta-analyses to detect **publication bias** or small study effects. - It plots the study effect size against a measure of its precision (e.g., standard error), typically forming a triangular or funnel shape if no bias is present. *Forest plot* - A **forest plot** is a graphical display used in meta-analyses to illustrate the **results of individual studies** along with their pooled estimate. - Each study is represented by a square and a horizontal line indicating the effect size and its **confidence interval**, respectively.
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