Pharmacokinetics and Pharmacodynamics Practice Questions

Q: Which of the following can reverse one or more smooth muscle effects of circulating histamine in humans?

Adrenaline. **Explanation:** The correct answer is **Adrenaline** because it acts as a **physiological antagonist** to histamine. While histamine causes bronchoconstriction and vasodilation (leading to hypotension) by acting on $H_1$ receptors, adrenaline acts on different receptors ($\beta_2$ and $\alpha_1$) to produce the exact opposite physiological effects: bronchodilation and vasoconstriction. This makes adrenaline the drug of choice for anaphylaxis, as it rapidly reverses the life-threatening smooth muscle effects of histamine regardless of the receptor involved. **Analysis of Incorrect Options:** * **Granisetron:** This is a selective **$5-HT_3$ receptor antagonist** used primarily as an antiemetic. It has no significant effect on histamine receptors or histamine-induced smooth muscle contraction. * **Ranitidine:** This is an **$H_2$ receptor blocker**. While it blocks histamine-induced gastric acid secretion, $H_2$ receptors have a minimal role in smooth muscle contraction compared to $H_1$ receptors. It cannot reverse systemic effects like bronchospasm. * **Sumatriptan:** This is a **$5-HT_{1B/1D}$ agonist** used in migraines to cause cranial vasoconstriction. It does not antagonize histamine's actions on systemic smooth muscles. **NEET-PG High-Yield Pearls:** * **Physiological Antagonism:** Two drugs acting on different receptors to produce opposite effects on the same physiological system (e.g., Adrenaline vs. Histamine, Insulin vs. Glucagon). * **Pharmacological Antagonism:** Competitive or non-competitive binding to the *same* receptor (e.g., Atropine vs. Acetylcholine). * **Clinical Note:** In anaphylactic shock, adrenaline is administered **Intramuscularly (IM)** in a **1:1000** concentration. It is the only drug that can reverse the "distributive shock" and "respiratory distress" caused by massive histamine release.

Q: Two drugs having opposite actions on different receptors demonstrate which type of antagonism?

Psychological antagonism. **Explanation:** The question describes **Physiological (Functional) Antagonism**, often referred to in some contexts as "psychological" antagonism (though "physiological" is the standard pharmacological term). This occurs when two agonists act on **different receptors** and produce **opposite effects** on the same physiological system [1]. **Why Option A is Correct:** In physiological antagonism, the drugs do not compete for the same binding site. Instead, they activate distinct pathways that counteract each other. A classic example is **Histamine** (acting on $H_1$ receptors to cause bronchoconstriction) and **Adrenaline** (acting on $\beta_2$ receptors to cause bronchodilation). **Why the other options are incorrect:** * **B. Physical Antagonism:** This is based on the physical properties of the drugs (e.g., Charcoal adsorbing alkaloids in the gut). No receptors are involved. * **C. Competitive Antagonism:** The antagonist binds to the **same receptor** as the agonist. This can be overcome by increasing the concentration of the agonist (surmountable). * **D. Non-competitive Antagonism:** The antagonist binds to an **allosteric site** or irreversibly to the active site of the same receptor, preventing the agonist from triggering a response regardless of concentration. **High-Yield Clinical Pearls for NEET-PG:** * **Chemical Antagonism:** Occurs when two substances react chemically to form an inactive product (e.g., Chelating agents like EDTA for lead poisoning or Protamine sulfate for Heparin). * **Adrenaline in Anaphylaxis:** This is the most high-yield example of **Physiological Antagonism**. Adrenaline counteracts the massive histamine release by acting on different receptors ($\alpha$ and $\beta$) to raise blood pressure and dilate airways. * **Key Distinction:** If the question mentions "same receptor," think Competitive/Non-competitive. If it mentions "different receptors/opposite effects," think Physiological.

Q: 5HT3 receptors belong to which of the following group of receptors?

Fastest acting receptors. **Explanation:** The correct answer is **B. Fastest acting receptors.** **Mechanism and Classification:** 5HT3 receptors are unique among serotonin receptors. While all other serotonin receptors (5HT1, 2, 4, 5, 6, 7) are G-protein coupled receptors (GPCRs), the **5HT3 receptor is a ligand-gated ion channel (ionotropic receptor).** Ionotropic receptors allow the direct flow of ions (sodium and potassium) across the cell membrane upon ligand binding, leading to rapid depolarization. This process occurs within milliseconds, making them the **fastest-acting** class of receptors in the body. **Analysis of Options:** * **A. Metabotropic:** These are GPCRs that act via second messengers (like cAMP or IP3/DAG). Their response takes seconds to minutes. All other 5HT receptors are metabotropic, but 5HT3 is not. * **C. Slowest acting receptors:** Nuclear receptors (which regulate gene transcription) are the slowest, taking hours to days. * **D. Enzymatic receptors:** These are transmembrane proteins with intrinsic enzymatic activity (e.g., Tyrosine Kinase receptors like the Insulin receptor). They are slower than ionotropic receptors. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** 5HT3 receptors are predominantly found in the **Chemoreceptor Trigger Zone (CTZ)** and the gastrointestinal tract (vagal afferents). * **Clinical Use:** 5HT3 antagonists (e.g., **Ondansetron, Palonosetron**) are the drugs of choice for **Chemotherapy-Induced Nausea and Vomiting (CINV)** and post-operative vomiting. * **Other Ionotropic Receptors:** Remember the mnemonic "N-G-A-5" for major ionotropic receptors: **N**icotinic (ACh), **G**ABA-A, **A**MPA/NMDA (Glutamate), and **5**HT3.

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.

Question 1

Which of the following drugs can be accumulated in the fetus in a very significant amount if given to the pregnant mother?

Accepted Answer

Lignocaine

Answer

Thiopentone

Answer

Propofol

Answer

Midazolam

Question 2

A three-year-old child presents to the emergency department after ingesting a large overdose of an antihistaminic drug. This drug is a weak base capable of entering most tissues, including the brain. Physical examination reveals a heart rate of 100/minute, blood pressure of 110/60 mm Hg, and a respiratory rate of 20/minute. In this case of poisoning, which of the following is true?

Accepted Answer

Urinary excretion would be accelerated by administration of NH4Cl, an acidifying agent.

Answer

Urinary excretion would be accelerated by administration of NaHCO3, an alkalinizing agent.

Answer

More of the drug would be ionized at blood pH than at stomach pH.

Answer

Absorption of the drug would be faster from the stomach than from the small intestine.

Question 3

Which of the following can reverse one or more smooth muscle effects of circulating histamine in humans?

Accepted Answer

Adrenaline

Answer

Granisetron

Answer

Ranitidine

Answer

Sumatriptan

Question 4

Which of the following statements is false?

Accepted Answer

Loading dose is equal to the volume of distribution multiplied by the target plasma concentration.

Answer

Lipid-soluble drugs tend to have a higher volume of distribution.

Answer

The volume of distribution is reduced by high plasma protein binding.

Answer

Half-life is constant in first-order kinetics.

Question 5

Two drugs having opposite actions on different receptors demonstrate which type of antagonism?

Accepted Answer

Psychological antagonism

Answer

Physical antagonism

Answer

Competitive antagonism

Answer

Non-competitive antagonism

Question 6

5HT3 receptors belong to which of the following group of receptors?

Accepted Answer

Fastest acting receptors

Answer

Metabotropic

Answer

Slowest acting receptors

Answer

Enzymatic receptors

Question 7

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 8

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 9

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 10

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.

Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics — MCQs

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