Pharmacokinetics and Pharmacodynamics Practice Questions

Q: Slow acetylators of isoniazid are more prone to develop which of the following?

Peripheral neuropathy. **Explanation:** The metabolism of **Isoniazid (INH)** occurs primarily in the liver via **N-acetylation** by the enzyme **N-acetyltransferase 2 (NAT2)**. This process is genetically determined, leading to two distinct phenotypes: fast acetylators and slow acetylators. **Why Peripheral Neuropathy is Correct:** In **slow acetylators**, the rate of metabolism is reduced, leading to higher plasma concentrations of Isoniazid. Isoniazid promotes the excretion of **Pyridoxine (Vitamin B6)** and inhibits the enzyme pyridoxine kinase. Higher levels of the drug result in a more profound deficiency of Vitamin B6, which is essential for myelin sheath integrity. This leads to **peripheral neuropathy**. To prevent this, 10–50 mg/day of Pyridoxine is co-administered with INH. **Analysis of Incorrect Options:** * **A. Failure of therapy:** This is more common in **fast acetylators** because the drug is metabolized and cleared too quickly, potentially falling below the therapeutic window. * **C. Hepatotoxicity:** While Isoniazid causes liver injury, it is traditionally associated more with **fast acetylators**. This is because fast acetylation produces more **acetyl-hydrazine**, a toxic metabolite that causes oxidative stress in hepatocytes. (Note: Recent studies show a complex relationship, but for NEET-PG, fast acetylation = hepatotoxicity; slow acetylation = neuropathy). * **D. Allergic reactions:** These are idiosyncratic and not directly linked to the acetylation status or the pharmacokinetic profile of the drug. **High-Yield Clinical Pearls for NEET-PG:** * **Genetic Polymorphism:** NAT2 deficiency is an example of pharmacogenetic variation. * **Other drugs metabolized by Acetylation:** Remember the mnemonic **SHIP** (Sulfonamides, Hydralazine, Isoniazid, Procainamide). * **Drug-Induced Lupus:** Slow acetylators are also at a higher risk of developing Drug-Induced Lupus Erythematosus (DILE) when taking Hydralazine or Procainamide.

Q: A drug is administered in a dose of 200 mg and its plasma concentration is 40 microgram/ml. What is the volume of distribution?

5 litres. ### Explanation **1. Understanding the Correct Answer (A: 5 Litres)** The **Volume of Distribution ($V_d$)** is a theoretical volume that relates the total amount of drug in the body to the concentration of the drug in the plasma. It is calculated using the formula: $$V_d = \frac{\text{Total Amount of Drug (Dose)}}{\text{Plasma Concentration (C)}}$$ **Calculation:** * **Dose:** 200 mg * **Plasma Concentration:** 40 µg/ml. To maintain unit consistency, convert µg/ml to mg/L (Note: 1 µg/ml = 1 mg/L). Therefore, 40 µg/ml = 40 mg/L. * $V_d = \frac{200\text{ mg}}{40\text{ mg/L}} = \mathbf{5\text{ Litres}}$ A $V_d$ of 5L suggests the drug is primarily confined to the vascular compartment (plasma volume). **2. Analysis of Incorrect Options** * **B (0.5 L):** This value is too low for a human adult; even plasma volume is approximately 3L. This result occurs if one incorrectly divides 20 by 40. * **C (2.5 L) & D (3 L):** These values do not satisfy the mathematical ratio of the given dose and concentration. A $V_d$ of 3L would represent the average plasma volume, but it does not fit the specific parameters of this question. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Low $V_d$ ( 42L):** Drug sequesters in tissues/fat (e.g., **Digoxin, Chloroquine**). These drugs cannot be removed by hemodialysis during toxicity. * **Loading Dose:** $V_d$ is the primary determinant used to calculate the Loading Dose ($LD = V_d \times \text{Target } C_p$).

Q: Which of the following is FALSE regarding sustained-release tablets?

They have less chances of toxicity.. **Explanation:** Sustained-release (SR) or extended-release formulations are designed to maintain therapeutic drug concentrations for a prolonged period. **Why Option B is FALSE (The Correct Answer):** Contrary to common belief, SR tablets actually carry a **higher risk of toxicity** if the delivery mechanism fails. This phenomenon is known as **"Dose Dumping,"** where the entire large dose intended for 12–24 hours is released prematurely into the systemic circulation. This can lead to fatal toxicity, especially in drugs with a narrow therapeutic index. Therefore, saying they have "less chances of toxicity" is clinically incorrect. **Analysis of Other Options:** * **Option A:** The goal of SR formulations is to provide **Zero-order kinetics**, releasing the drug at a constant rate independent of the amount remaining in the tablet to maintain steady-state plasma levels. * **Option C:** To provide therapeutic effects over an extended duration (e.g., 24 hours), these tablets must contain a **significantly higher total dose** of the drug compared to immediate-release versions. * **Option D:** Because of the risk of dose dumping, the **margin of safety (Therapeutic Index)** is a critical consideration. Drugs with a very narrow therapeutic window (e.g., Digoxin) are generally not preferred for SR formulations unless the delivery technology is exceptionally robust. **High-Yield NEET-PG Pearls:** * **Zero-order kinetics:** Rate of elimination/release is constant (independent of concentration). * **Dose Dumping:** Triggered by crushing the tablet or, in some cases, consuming alcohol with the medication. * **Contraindication:** SR tablets should **never be crushed or chewed**, as this destroys the release matrix. * **Ideal Candidates:** Drugs with short half-lives ($t_{1/2} < 4$ hours) are the best candidates for SR formulations to improve patient compliance.

Q: Which drug is highly distributed to body fat?

Thiopentone. ### Explanation The correct answer is **Thiopentone**. **1. Why Thiopentone is correct:** The distribution of a drug depends on its lipid solubility and ionization. Thiopentone is an ultra-short-acting barbiturate characterized by **extreme lipid solubility**. Upon intravenous administration, it rapidly crosses the blood-brain barrier to induce anesthesia. However, its action is terminated not by metabolism, but by **redistribution**. It moves from the highly vascular brain to less vascular but lipid-rich tissues like skeletal muscle and, eventually, **adipose tissue (body fat)**. Because fat has low blood flow, the drug accumulates there, acting as a reservoir. **2. Why the other options are incorrect:** * **Digoxin:** While it has a large volume of distribution ($V_d$), it primarily binds to **skeletal muscle** (via Na+/K+ ATPase) and heart tissue, not fat. * **Heparin:** This is a large, highly ionized (polar) molecule. It remains confined to the **plasma compartment** ($V_d \approx$ plasma volume) and does not distribute into tissues or fat. * **Mannitol:** It is a highly water-soluble (hydrophilic) sugar alcohol. It remains in the **extracellular fluid (ECF)** and does not cross cell membranes or distribute into adipose tissue. **3. NEET-PG High-Yield Pearls:** * **Redistribution:** This is the hallmark of highly lipid-soluble drugs (e.g., Thiopentone, Propofol). It explains why a patient wakes up quickly despite the drug still being present in the body. * **Volume of Distribution ($V_d$):** * Low $V_d$ (confined to plasma): Heparin, Warfarin. * Medium $V_d$ (ECF): Mannitol, Aminoglycosides. * High $V_d$ (Tissue binding): Digoxin, Chloroquine (highest $V_d$). * **Clinical Note:** In obese patients, Thiopentone can have a prolonged recovery time due to extensive accumulation in the large fat reservoir.

Q: Which of the following properties of a drug will enable it to be used in low concentration?

High affinity. **Explanation:** The correct answer is **High affinity**. **1. Why High Affinity is Correct:** Affinity refers to the chemical force that causes a drug to bind to its receptor. It is the measure of how tightly a drug binds to its target. In pharmacological terms, affinity is inversely proportional to the **Dissociation Constant ($K_d$)**. A drug with high affinity requires a very low concentration to occupy 50% of the available receptors. Therefore, drugs with high affinity can achieve their desired pharmacological effect even at low doses or concentrations. **2. Why Other Options are Incorrect:** * **High Specificity:** Specificity refers to the ability of a drug to act on a particular receptor type or tissue. While highly specific drugs have fewer side effects, specificity does not dictate the *amount* or *concentration* of the drug needed to trigger a response. * **Low Specificity:** This implies the drug binds to multiple receptor types (promiscuity), which usually leads to a higher incidence of adverse effects, but it does not determine the potency or concentration required for action. * **High Stability:** This relates to the drug's resistance to degradation (metabolism) and its shelf-life. While stability affects the duration of action and storage, it does not determine the concentration needed to initiate a receptor-mediated response. **3. NEET-PG High-Yield Pearls:** * **Potency vs. Efficacy:** Affinity is a major determinant of **Potency** (the amount of drug needed for an effect). **Efficacy** (Intrinsic Activity) is the ability of the drug to activate the receptor once bound. * **$K_d$ Relationship:** A lower $K_d$ value signifies higher affinity. * **Agonists vs. Antagonists:** Both agonists and antagonists possess affinity, but only agonists possess intrinsic activity (efficacy).

Q: Loading dose primarily depends on which pharmacokinetic parameter?

Volume of distribution. **Explanation:** The **Loading Dose (LD)** is the initial higher dose of a drug given at the beginning of a course of treatment before dropping to a lower maintenance dose. Its primary purpose is to rapidly achieve the **target therapeutic plasma concentration ($C_p$)**. **1. Why Volume of Distribution ($V_d$) is correct:** The loading dose is mathematically defined by the formula: $$\text{Loading Dose} = \frac{V_d \times \text{Target } C_p}{\text{Bioavailability (F)}}$$ Since the goal is to "fill up" the body's various compartments (tissues and plasma) to reach a specific concentration, the amount of drug required is directly proportional to the space it occupies ($V_d$). If a drug has a high $V_d$ (extensive tissue binding), a larger loading dose is needed. **2. Why the other options are incorrect:** * **Clearance (B):** This determines the **Maintenance Dose**. Clearance relates to how fast the drug is removed, which dictates how much must be replaced to maintain a steady state. * **Rate of administration (C):** This affects the peak plasma concentration and potential toxicity (especially with IV bolus) but does not determine the total dose required to reach steady state. * **Half-life (D):** This determines the **time taken** to reach steady state (usually 4–5 half-lives) and the dosing interval, but not the magnitude of the loading dose itself. **High-Yield Clinical Pearls for NEET-PG:** * **Loading Dose:** "Fills the tank" (Depends on $V_d$). * **Maintenance Dose:** "Replaces the leak" (Depends on Clearance). * **Clinical Example:** Digoxin and Phenytoin often require loading doses due to their large $V_d$ and the need for rapid effect. * **Note:** In patients with renal or hepatic failure, the loading dose usually remains the same (unless $V_d$ is altered), but the maintenance dose must be decreased.

Question 1

Which of the following is true regarding drugs that are highly bound to albumin?

Accepted Answer

They can undergo competition with other drugs for albumin binding sites.

Answer

They effectively cross the blood-brain barrier.

Answer

They have a large volume of distribution.

Answer

They are easily filtered at the glomerulus.

Question 2

Slow acetylators of isoniazid are more prone to develop which of the following?

Accepted Answer

Peripheral neuropathy

Answer

Failure of therapy

Answer

Hepatotoxicity

Answer

Allergic reactions

Question 3

A drug is administered in a dose of 200 mg and its plasma concentration is 40 microgram/ml. What is the volume of distribution?

Accepted Answer

5 litres

Answer

0.5 litres

Answer

2.5 litres

Answer

3 litres

Question 4

Which of the following is FALSE regarding sustained-release tablets?

Accepted Answer

They have less chances of toxicity.

Answer

They release the drug at a constant rate.

Answer

They contain a large amount of the drug.

Answer

The margin of safety is an important consideration for their formulation.

Question 5

Which of the following is characteristic of drugs exhibiting zero-order kinetics of elimination?

Accepted Answer

Their elimination half-life is independent of the dose.

Answer

They are more common than drugs following first-order kinetics.

Answer

Their concentration decreases exponentially with time.

Answer

A plot of drug concentration versus time is linear.

Question 6

Plasma concentrations (g/mL) after intravenous administration of 5.1 mg/kg of Gentamicin to a hypothetical patient are depicted in the graph. The red color in the graph represents the dose administered at which time point?

Accepted Answer

24 hours

Answer

12 hours

Answer

8 hours

Answer

Threshold

Question 7

Which drug is highly distributed to body fat?

Accepted Answer

Thiopentone

Answer

Digoxin

Answer

Heparin

Answer

Mannitol

Question 8

Which of the following properties of a drug will enable it to be used in low concentration?

Accepted Answer

High affinity

Answer

High specificity

Answer

Low specificity

Answer

High stability

Question 9

Procainamide infusion is initiated. Its half-life is 2 hours. The infusion begins at 9:00 a.m. and ends at 1:00 p.m. on the same day. At 1:00 p.m., the blood concentration is found to be 3 mg/L. What is the probable steady-state concentration after 2 days of infusion?

Accepted Answer

4 mg/L

Answer

3 mg/L

Answer

6 mg/L

Answer

15 mg/L

Question 10

Loading dose primarily depends on which pharmacokinetic parameter?

Accepted Answer

Volume of distribution

Answer

Clearance

Answer

Rate of administration

Answer

Half-life

Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics — MCQs

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