Pharmacokinetics and Pharmacodynamics Practice Questions

Q: Rate of absorption of an intramuscular injection is determined by which of the following factors?

Vascularity. **Explanation:** The rate of absorption of a drug from an intramuscular (IM) site is primarily determined by the **vascularity** (blood flow) of the muscle. 1. **Why Vascularity is Correct:** Absorption from an IM site occurs via simple diffusion from the muscle interstitial fluid into the capillaries. The rate-limiting step for small, water-soluble molecules is the **blood flow** to the area. Muscles with higher vascularity (e.g., Deltoid) exhibit faster absorption rates compared to those with lower vascularity (e.g., Gluteus maximus). This is also why exercise, which increases regional blood flow, accelerates IM drug absorption. 2. **Why Other Options are Incorrect:** * **Molecular Weight:** While it affects the *mechanism* of absorption (large molecules enter via lymphatics), the primary determinant of the *rate* in clinical IM administration is perfusion, not the size of the molecule. * **Volume of the Drug:** Volume primarily affects the pressure at the injection site and potential tissue damage/pain, but it does not dictate the physiological rate of systemic uptake. * **Bore of the Needle:** This determines the ease of administration (viscosity management) and patient discomfort, but has no physiological impact on how the drug crosses the capillary membrane once deposited. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Absorption Rate:** Deltoid > Vastus lateralis > Gluteus maximus (due to decreasing vascularity). * **Depot Preparations:** Drugs like Penicillin G Benzathine or Haloperidol Decanoate are designed as insoluble salts in oil to *intentionally* slow down absorption despite high vascularity. * **Shock/Hypotension:** In patients with circulatory collapse, IM absorption is significantly impaired due to peripheral vasoconstriction; hence, the **Intravenous (IV)** route is preferred in emergencies.

Q: All of the following are Phase II reactions except?

Cyclization. **Explanation:** Drug metabolism (biotransformation) typically occurs in two phases. **Phase I (Nonsynthetic) reactions** involve the introduction or unmasking of a functional group (–OH, –NH2, –SH), while **Phase II (Synthetic) reactions** involve the attachment of an endogenous moiety to the drug to make it more water-soluble for excretion. **Why Cyclization is the correct answer:** Cyclization is a **Phase I reaction**. It involves the formation of a ring structure from an open-chain compound (e.g., proguanil to cycloguanil). Other Phase I reactions include Oxidation (most common), Reduction, Hydrolysis, and Decarboxylation. **Analysis of Incorrect Options (Phase II Reactions):** Phase II reactions involve **conjugation**. * **A. Acetylation:** A Phase II reaction mediated by N-acetyltransferase (e.g., Isoniazid, Hydralazine). * **B. Methylation:** A Phase II reaction (e.g., Adrenaline to Metanephrine). * **C. Glycine conjugation:** A Phase II reaction (e.g., Salicylates conjugated with glycine to form hippuric acid). * *Other Phase II reactions include Glucuronidation (most common), Sulfation, and Glutathione conjugation.* **High-Yield NEET-PG Pearls:** 1. **Glucuronidation** is the most common Phase II reaction and the only one that occurs in the microsomal fraction (smooth ER); others are non-microsomal. 2. **Oxidation** is the most common Phase I reaction, primarily mediated by **Cytochrome P450** enzymes. 3. **Phase II before Phase I:** While the sequence is usually I then II, some drugs like **Isoniazid** undergo Phase II (Acetylation) followed by Phase I (Hydrolysis). 4. **Slow Acetylators:** Genetic polymorphism in acetylation (NAT2 enzyme) can lead to toxicity with drugs like Isoniazid (peripheral neuropathy) and Hydralazine (Lupus-like syndrome).

Q: An antagonist that generates an action opposite to a substance by binding to different receptors is known as:

Physiological antagonist. ### Explanation **1. Why Physiological Antagonist is Correct:** A **physiological (or functional) antagonist** occurs when two drugs act on **different receptors** and produce **oppositely directed** physiological effects on the same system. Unlike pharmacological antagonism, there is no competition for the same receptor site. Instead, the drugs activate independent pathways that cancel each other out functionally. * *Classic Example:* **Histamine** (acts on H1 receptors to cause bronchoconstriction) vs. **Adrenaline** (acts on β2 receptors to cause bronchodilation). **2. Why Other Options are Incorrect:** * **Physical Antagonist (A):** Based on the physical property of the drug. For example, **Charcoal** adsorbs alkaloids, preventing their absorption. * **Chemical Antagonist (B):** Two substances react chemically to form an inactive product. No receptors are involved. For example, **Chelating agents** (EDTA) binding to heavy metals or **Antacids** neutralizing gastric acid. * **Pharmacologic Antagonist (D):** The antagonist binds to the **same receptor** as the agonist, preventing the agonist from binding. This can be competitive (reversible) or non-competitive (irreversible). **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Adrenaline in Anaphylaxis:** Adrenaline is the "physiological antagonist" of histamine and is the drug of choice for anaphylactic shock because it rapidly reverses life-threatening airway obstruction and hypotension via different receptors. * **Glucagon and Insulin:** These are physiological antagonists regarding blood glucose levels (Glucagon increases glucose via glucagon receptors; Insulin decreases it via insulin receptors). * **Key Distinction:** If the question mentions "same receptor," think **Pharmacologic**; if it mentions "different receptor/same system," think **Physiological**.

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.

Question 1

An intravenous bolus dose of thiopentone leads to loss of consciousness within 10-15 seconds. The patient regains consciousness in just a few minutes. This is because it is:

Accepted Answer

Redistributed from brain to other body tissues

Answer

Renally excreted

Answer

Exhaled rapidly

Answer

Rapidly metabolized by hepatic enzymes

Question 2

Rate of absorption of an intramuscular injection is determined by which of the following factors?

Accepted Answer

Vascularity

Answer

Molecular weight of the drug

Answer

Volume of the drug

Answer

Bore of the needle

Question 3

All of the following are Phase II reactions except?

Accepted Answer

Cyclization

Answer

Acetylation

Answer

Methylation

Answer

Glycine conjugation

Question 4

An antagonist that generates an action opposite to a substance by binding to different receptors is known as:

Accepted Answer

Physiological antagonist

Answer

Physical antagonist

Answer

Chemical antagonist

Answer

Pharmacologic antagonist

Question 5

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 6

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 7

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 8

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 9

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 10

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

Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics — MCQs

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