Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics Indian Medical PG Practice Questions and MCQs

Question 551: Which of the following statements about warfarin is false?

A. Half-life is 25 to 60 hours
B. Inhibits vitamin K dependent clotting factor synthesis
C. Warfarin does not cross the placenta (Correct Answer)
D. Contraindicated in patients with severe hepatic failure

Explanation: ***Warfarin does not cross the placenta*** - Warfarin **rapidly crosses the placenta**, leading to fetal exposure and potential teratogenic effects, such as **fetal warfarin syndrome**. - This characteristic makes warfarin generally **contraindicated during pregnancy**, especially in the first trimester. *Half-life is 25 to 60 hours* - The half-life of warfarin is indeed relatively long, ranging from approximately **25 to 60 hours**, which contributes to its once-daily dosing. - This prolonged half-life means that changes in dosing take several days to reach a **new steady-state** and affect the International Normalized Ratio (INR). *Inhibits vitamin K dependent clotting factor synthesis* - Warfarin functions as a **vitamin K antagonist**, inhibiting the hepatic synthesis of vitamin K-dependent clotting factors **II, VII, IX, and X**. - It achieves this by blocking **vitamin K epoxide reductase**, an enzyme essential for reactivating vitamin K. *Contraindicated in patients with severe hepatic failure* - Warfarin is primarily metabolized in the **liver** by cytochrome P450 enzymes, and its clotting factors are synthesized in the liver. - Therefore, **severe hepatic failure** can impair both warfarin metabolism and the production of clotting factors, leading to an increased risk of bleeding and making it a contraindication.

Question 552: A 40-year-old patient on oxygen therapy has developed digoxin toxicity. The plasma digoxin level is 4 ng/mL. Renal function is normal, and the plasma half-life of digoxin in this patient is 1.6 days. How long should digoxin be withheld to reach a safer yet probably therapeutic level of 1 ng/mL?

A. 1.6 days
B. 2.4 days
C. 3.2 days (Correct Answer)
D. 4.8 days

Explanation: ***3.2 days*** - To reach 1 ng/mL from 4 ng/mL, the digoxin level must undergo two half-lives (4 ng/mL → 2 ng/mL → 1 ng/mL). - Given a half-life of 1.6 days, two half-lives would be 1.6 days * 2 = **3.2 days**. *1.6 days* - This represents only one half-life, which would reduce the digoxin level from 4 ng/mL to 2 ng/mL, not the target of 1 ng/mL. - The patient would still be above the desired therapeutic level, and likely still experiencing toxicity. *2.4 days* - This period is equivalent to 1.5 half-lives (1.6 days * 1.5 = 2.4 days), which would reduce the digoxin level to 1.41 ng/mL. - This level is still higher than the desired 1 ng/mL, and while lower, may not fully resolve the toxicity. *4.8 days* - This period represents three half-lives (1.6 days * 3 = 4.8 days), which would reduce the digoxin level to 0.5 ng/mL. - While safer, this level may be sub-therapeutic and unnecessarily prolong the withholding of the drug, delaying the re-establishment of therapeutic effects.

Question 553: What are the primary mechanisms involved in the termination of drug action?

A. Drugs must be eliminated from the body to terminate their action.
B. Metabolism of drugs can either activate or abolish their pharmacologic activity.
C. Hepatic metabolism and renal excretion are key mechanisms in drug action termination. (Correct Answer)
D. Distribution of a drug from the bloodstream can influence its effects.

Explanation: ***Hepatic metabolism and renal excretion are key mechanisms in drug action termination.*** [1], [2] - These represent the **two most important primary mechanisms** for terminating drug action in the body - **Hepatic metabolism** (Phase I and Phase II reactions) converts lipophilic drugs into more polar, water-soluble metabolites, often inactivating them pharmacologically [1], [4] - **Renal excretion** is the principal route of elimination for both unchanged drugs and their metabolites [5] - Together, these account for the majority of drug clearance from the body and are the foundation of pharmacokinetic principles [3] - This option directly and comprehensively answers what the "primary mechanisms" are *Drugs must be eliminated from the body to terminate their action.* - While this statement is **true as a general principle**, it describes the **necessity** of elimination rather than specifying the actual **mechanisms** involved - This is too vague to answer a question asking for specific primary mechanisms - It doesn't identify which elimination processes are most important (metabolism, excretion, redistribution, etc.) *Metabolism of drugs can either activate or abolish their pharmacologic activity.* - This is factually correct but describes the **outcomes or consequences** of metabolism, not the mechanisms of drug action termination [4] - This focuses on bidirectional effects (activation vs inactivation) rather than specifically addressing termination mechanisms - Prodrugs are activated (e.g., enalapril → enalaprilat), while most active drugs are inactivated, but this doesn't answer which mechanisms primarily terminate drug action [4] *Distribution of a drug from the bloodstream can influence its effects.* - **Redistribution** can contribute to termination of drug effect (e.g., ultra-short acting barbiturates like thiopental) - However, redistribution is a **secondary or temporary mechanism** that moves drug away from the site of action to other tissues - It doesn't represent a **primary mechanism** of ultimate drug elimination from the body - This is more relevant for understanding duration of action rather than the main termination pathways

Question 554: Zero-order kinetics is seen in all except which of the following drugs?

A. Phenytoin
B. High dose salicylates
C. Methotrexate (Correct Answer)
D. Ethanol

Explanation: ***Methotrexate*** - Methotrexate typically exhibits **first-order kinetics** at therapeutic doses, meaning its elimination rate is proportional to its concentration. - While high doses or impaired renal function could theoretically lead to saturation, its usual elimination follows **first-order principles**. *Phenytoin* - Phenytoin exhibits **zero-order kinetics** at therapeutic concentrations due to saturation of its metabolic enzymes (e.g., CYP2C9, CYP2C19) in the liver. - This means a **constant amount** of the drug is eliminated per unit of time, irrespective of its plasma concentration. *High dose salicylates* - At **high doses**, the metabolic pathways for salicylates (like aspirin) become saturated, leading to **zero-order kinetics**. - This saturation means a **fixed amount** of salicylate is eliminated over time, increasing the risk of toxicity with dose increments. *Ethanol* - Ethanol metabolism, primarily by **alcohol dehydrogenase**, follows **zero-order kinetics** at most concentrations relevant to consumption. - A **constant amount** of ethanol is eliminated per unit of time, regardless of how much is present in the body.

Question 555: Which opioid drug is effectively administered via the transbuccal route?

A. Sulfentanil
B. Remifentanil
C. Fentanyl (Correct Answer)
D. Alfentanil

Explanation: ***Fentanyl*** - **Fentanyl** is a potent, **lipophilic opioid** that is well-absorbed through mucous membranes, making it suitable for **transbuccal administration**. - Its high potency and rapid onset of action when administered transbuccally make it useful for breakthrough pain or rapid analgesia. *Sulfentanil* - While also a potent opioid, **sulfentanil** is primarily used intravenously for anesthesia and is not commonly formulated or administered via the transbuccal route. - Its chemical properties and pharmacokinetic profile do not lend themselves as readily to transbuccal absorption compared to fentanyl for practical clinical use. *Remifentanil* - **Remifentanil** is an **ultra-short-acting opioid** metabolized by plasma esterases, making it ideal for continuous intravenous infusions where rapid offset is desired. - Its rapid metabolism and specific pharmacokinetic properties make it unsuitable for transbuccal extended release or sustained absorption. *Alfentanil* - **Alfentanil** is a short-acting opioid predominantly used intravenously for induction and maintenance of anesthesia. - Although it has a rapid onset, it is not optimized or commonly utilized for transbuccal administration due to its lower lipophilicity and different absorption characteristics compared to fentanyl.

Question 556: Which drug is metabolized by glutathionation?

A. Nicotinic acid
B. Fosfomycin
C. Benzodiazepines
D. Dapsone (Correct Answer)

Explanation: ***Dapsone***- **Dapsone** undergoes hepatic metabolism via **N-hydroxylation** by CYP450 enzymes (particularly CYP2E1 and CYP3A4), forming reactive **hydroxylamine metabolites**.- These reactive metabolites are toxic and can cause **methemoglobinemia** and **hemolysis**.- **Glutathione conjugation (glutathionation)** serves as an important **detoxification pathway** for these reactive dapsone metabolites [1].- Individuals with **glutathione deficiency** (such as G6PD deficiency) are at increased risk of dapsone-induced hemolytic anemia [2].*Fosfomycin*- **Fosfomycin** is primarily eliminated by the kidneys as an **unchanged drug** (up to 90% excreted unchanged in urine).- It undergoes **minimal hepatic metabolism** and does NOT undergo significant glutathionation.- Its primary route of elimination is **renal excretion** via glomerular filtration.*Benzodiazepines*- **Benzodiazepines** are primarily metabolized in the liver via **CYP450 enzymes** (Phase I oxidation) followed by **glucuronidation** (Phase II conjugation).- They do NOT undergo glutathionation as a significant metabolic pathway.*Nicotinic acid*- **Nicotinic acid** (niacin) undergoes conjugation with **glycine** to form nicotinuric acid and **methylation** to form N-methylnicotinamide.- It does NOT undergo glutathione conjugation.

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Absorption and Bioavailability

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Drug Distribution and Protein Binding

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Biotransformation and Metabolism Pathways

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Renal and Non-renal Excretion

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Compartment Models

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Dose-Response Relationships

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Drug Efficacy and Potency

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Drug Tolerance and Tachyphylaxis

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Population Pharmacokinetics

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Pharmacokinetic Variability

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