Pharmacokinetics and Pharmacodynamics Practice Questions

Q: What is the definition of bioavailability?

Ratio of Area of oral to Area of IV. ***Ratio of Area of oral to Area of IV*** - **Bioavailability (F)** is defined as the **fraction** of an administered dose of unchanged drug that reaches the systemic circulation - It is calculated as: **F = AUC_oral / AUC_IV** (when doses are equal) - This gives a value between **0 and 1** (e.g., F = 0.75 means 75% bioavailable) - This is the **standard pharmacological definition** as per Goodman & Gilman and other authoritative texts *Ratio of Area of oral to Area of IV multiplied by 100* - This expresses bioavailability as a **percentage** rather than a fraction - While bioavailability can be *communicated* as a percentage (e.g., "75% bioavailable"), the formal definition is as a **fraction** - Multiplying by 100 is a conversion for expression, not part of the core definition *Ratio of Area of IV to Area of oral* - This is the **inverse ratio** of bioavailability - It would give values **greater than 1** for drugs with incomplete absorption - Does not represent the fraction of oral dose reaching systemic circulation *Ratio of Area of IV to Area of oral multiplied by 100* - This combines two errors: **inverse ratio** and percentage conversion - Would yield values >100% for drugs with bioavailability <1 - Completely incorrect definition of bioavailability

Q: What percentage of a drug is eliminated after four half-lives in first-order kinetics?

93.75%. ***93.75%*** - In **first-order kinetics**, a constant **fraction** (not amount) of the drug is eliminated per unit time - After 1 half-life: 50% eliminated (50% remains) - After 2 half-lives: 75% eliminated (25% remains) - After 3 half-lives: 87.5% eliminated (12.5% remains) - After 4 half-lives: **93.75% eliminated** (6.25% remains) *84.00%* - This represents approximately 16% remaining drug - Would occur between the 2nd and 3rd half-lives, not at exactly 4 half-lives - Does not correspond to the mathematical progression of first-order kinetics *80.00%* - This represents 20% remaining drug - Does not correspond to any whole number of half-lives - Falls between 2 half-lives (75% eliminated) and 3 half-lives (87.5% eliminated) *5.00%* - This suggests only 5% elimination with 95% drug remaining - Would occur very early in the elimination process, less than 1 half-life - Fundamentally incorrect for 4 half-lives where 93.75% should be eliminated

Q: Which of the following factors does not affect the action of lignocaine?

Action of cholinesterase at the site of injection. ***Action of cholinesterase at the site of injection*** - **Lidocaine** is an **amide-type local anesthetic** and is primarily metabolized by the liver, not by cholinesterases. - Cholinesterases are responsible for metabolizing **ester-type local anesthetics** (e.g., procaine) and acetylcholine. *PH at the site of injection* - The **pH** at the injection site significantly affects lignocaine's action, as its **unionized form** (which is favored in alkaline pH) is essential for penetrating nerve membranes. - In acidic environments (e.g., infected tissue), less unionized lignocaine is available, leading to reduced efficacy. *Blood flow at the site of injection* - **Increased blood flow** at the site of injection can accelerate the systemic absorption of lignocaine, thereby reducing its duration of action at the nerve. - **Decreased blood flow** can prolong its local action, but also increase the risk of systemic toxicity if absorption is very slow. *Vasoconstrictor in the L.A solution* - The addition of a **vasoconstrictor** (e.g., epinephrine) slows the systemic absorption of lignocaine from the injection site. - This **prolongs the duration of action** and reduces the risk of systemic toxicity by keeping the anesthetic concentrated at the target nerves.

Q: Which of the following drugs is least likely to cross the blood-placental barrier?

Glycopyrrolate. ***Glycopyrrolate*** - Glycopyrrolate is a **quaternary ammonium compound**, meaning it is highly ionized and has a low lipid solubility. - Its **polar nature** and **larger molecular weight** restrict its ability to readily cross lipid membranes, including the blood-placental barrier. *Atropine* - **Atropine** is a tertiary amine, making it a **lipid-soluble** compound. - Its lipid solubility allows it to **easily cross the blood-placental barrier** and affect the fetus. *Physostigmine* - Physostigmine is also a **tertiary amine** and is relatively **lipid-soluble**. - This property enables it to **readily cross lipid barriers** such as the blood-placental barrier and the blood-brain barrier. *Hyoscine hydrobromide* - **Hyoscine hydrobromide** (scopolamine) is a **tertiary amine** and highly **lipid-soluble**. - Its high lipid solubility allows for its **easy passage across the blood-placental barrier** and into the fetal circulation.

Q: 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?

3.2 days. ***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.

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

Methotrexate. ***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 1

What is the definition of bioavailability?

Accepted Answer

Ratio of Area of oral to Area of IV

Answer

Ratio of Area of IV to Area of oral

Answer

Ratio of Area of IV to Area of oral multiplied by 100

Answer

Ratio of Area of oral to Area of IV multiplied by 100

Question 2

What percentage of a drug is eliminated after four half-lives in first-order kinetics?

Accepted Answer

93.75%

Answer

84.00%

Answer

80.00%

Answer

5.00%

Question 3

Which of the following factors does not affect the action of lignocaine?

Accepted Answer

Action of cholinesterase at the site of injection

Answer

PH at the site of injection

Answer

Vasoconstrictor in the L.A solution

Answer

Blood flow at the site of injection

Question 4

What is the mechanism by which standard doses of lidocaine can lead to cardiac or central nervous system toxicity in patients with circulatory failure?

Accepted Answer

Lidocaine concentration can increase in well-perfused tissues such as the brain and heart during circulatory failure.

Answer

Histamine receptors in the brain and heart are activated during circulatory failure.

Answer

There is a sudden release of catecholamines in the brain and heart during circulatory failure.

Answer

Lidocaine is metabolized in the liver, but this does not lead to a toxic metabolite.

Question 5

Which of the following drugs is least likely to cross the blood-placental barrier?

Accepted Answer

Glycopyrrolate

Answer

Atropine

Answer

Physostigmine

Answer

Hyoscine hydrobromide

Question 6

Which of the following statements is true about first-order kinetics?

Accepted Answer

The rate of elimination is proportional to the plasma concentration.

Answer

The half-life increases with an increase in dose.

Answer

A constant amount is eliminated in unit time.

Answer

The elimination follows zero-order kinetics at therapeutic doses.

Question 7

All of the following statements regarding the bioavailability of a drug are true except which one?

Accepted Answer

Low oral availability always and necessarily means poor absorption

Answer

It is a fraction of administered drug that reaches the systemic circulation in unchanged form

Answer

Bioavailability can be determined from plasma concentration or urinary excretion data.

Answer

Bioavailability of an orally administered drug can be calculated by comparing the Area Under Curve after oral and intravenous administration

Question 8

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?

Accepted Answer

3.2 days

Answer

1.6 days

Answer

2.4 days

Answer

4.8 days

Question 9

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

Accepted Answer

Hepatic metabolism and renal excretion are key mechanisms in drug action termination.

Answer

Drugs must be eliminated from the body to terminate their action.

Answer

Metabolism of drugs can either activate or abolish their pharmacologic activity.

Answer

Distribution of a drug from the bloodstream can influence its effects.

Question 10

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

Accepted Answer

Methotrexate

Answer

Phenytoin

Answer

High dose salicylates

Answer

Ethanol

Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics — MCQs

On this page

Practice by Chapter

Want unlimited practice?