Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics Indian Medical PG Practice Questions and MCQs

Question 381: Pharmacodynamics deals with:-

A. Latency of onset
B. Mechanism of action of a drug (Correct Answer)
C. Transport of drug across the biological membranes
D. Mode of excretion of a drug

Explanation: Detailed study of the **Mechanism of action of a drug** [1][2] - **Pharmacodynamics** describes what the **drug does to the body**, including its **molecular targets** and biochemical effects [3]. - This involves the study of the drug's mechanisms to produce its therapeutic or toxic effects [2]. *Latency of onset* - **Latency of onset** refers to the time it takes for a drug to start producing its effects, which is a pharmacokinetic rather than a pharmacodynamic parameter. - It deals with the drug's absorption and distribution rather than its interaction with the body once it reaches its site of action. *Transport of drug across the biological membranes* - The **transport of drugs across biological membranes** is a key aspect of **pharmacokinetics**, specifically absorption and distribution [1]. - This process determines how much drug reaches its target site, not how it interacts with the target. *Mode of excretion of a drug* - The **mode of excretion** of a drug (e.g., renal, hepatic) falls under **pharmacokinetics**, addressing how the body gets rid of the drug. - This process influences the drug's duration of action and elimination half-life, not its mechanism of action.

Question 382: You are in the eye OPD and wish to use a topical Beta blocker in a patient. The chosen drug by you should have all the following properties except

A. High ocular capture
B. High lipophilicity
C. Low Systemic activity
D. Strong local anaesthetic activity (Correct Answer)

Explanation: ***Strong local anaesthetic activity*** - Topical beta blockers for glaucoma should **not possess significant local anesthetic activity** as this property is not related to their mechanism of action in lowering intraocular pressure and could lead to unwanted corneal effects or masking of pain. - Their primary role is to **reduce aqueous humor production** by blocking beta-adrenergic receptors on the ciliary epithelium. *High ocular capture* - **High ocular capture** (good penetration into the eye) is a desirable property for topical eye medications, ensuring sufficient drug concentration at the target tissues (e.g., ciliary body) to exert its therapeutic effect. - This property allows the drug to effectively inhibit aqueous humor production and **lower intraocular pressure**. *High lipophilicity* - **High lipophilicity** is beneficial for topical ophthalmic drugs as it enhances their ability to **cross lipid-rich corneal barriers** and reach the aqueous humor and ciliary body. - This property contributes to improved drug penetration and overall **ocular bioavailability**. *Low Systemic activity* - **Low systemic activity** is a crucial characteristic for topical ophthalmic drugs, especially beta blockers, to minimize systemic side effects such as **bradycardia**, **bronchospasm**, or **hypotension**. - Systemic absorption is reduced by limiting drug access to the general circulation, for example, by **nasolacrimal occlusion**.

Question 383: Which is true for glucuronidation:

A. Done by CYP enzyme
B. Phase I reaction
C. Phase II reaction (Correct Answer)
D. Water solubility is decreased

Explanation: ***Phase II reaction***- **Glucuronidation** is a major **Phase II** metabolic pathway that conjugates a glucuronic acid molecule to a **lipophilic substance** [1]- This process significantly increases the **water solubility** of the substance, facilitating its excretion from the body [1, 2]- Catalyzed by **UDP-glucuronosyltransferases (UGTs)**, a family of conjugation enzymes [1]*Done by CYP enzyme*- **CYP (cytochrome P450)** enzymes are primarily involved in **Phase I** metabolic reactions, which typically functionalize compounds through oxidation, reduction, or hydrolysis [1]- Glucuronidation is catalyzed by **UDP-glucuronosyltransferases (UGTs)**, not CYP enzymes [1]*Phase I reaction*- **Phase I reactions** (e.g., oxidation, reduction, hydrolysis) introduce or expose polar functional groups on xenobiotics [1]- Glucuronidation is a **conjugation reaction**, characteristic of **Phase II metabolism**, not Phase I [1]*Water solubility is decreased*- Glucuronidation involves the attachment of a **hydrophilic glucuronic acid** molecule to the substrate [1]- This conjugation significantly **increases the water solubility** of the metabolite, which is essential for its efficient urinary or biliary excretion [1, 2]- The addition of the polar glucuronic acid group makes lipophilic compounds more water-soluble [1]

Question 384: All of the following can be routes of opioid administration except:

A. Intramuscular
B. Oral
C. Intravenous
D. Intradermal (Correct Answer)

Explanation: ***Intradermal*** - **Intradermal administration** involves injecting medication into the dermis, the layer between the epidermis and the subcutaneous tissue, and is typically used for **allergy testing** or **tuberculosis screening (PPD test)**, not for systemic opioid delivery. - The **slow absorption rate** and **small volume capacity** of the dermal layer make it unsuitable for achieving therapeutic opioid concentrations quickly or effectively. *Intramuscular* - **Intramuscular (IM)** injection allows for **rapid absorption** of opioids into the bloodstream from the muscle tissue. - It is a common route for administering **analgesics**, including opioids, especially in settings where oral administration is not feasible or faster onset is desired. *Oral* - **Oral (PO) administration** is a common and convenient route for many opioid formulations, allowing for **systemic absorption** through the gastrointestinal tract. - Opioids like **oxycodone**, **hydrocodone**, and **morphine** are often prescribed as oral tablets or solutions for pain management. *Intravenous* - **Intravenous (IV) administration** provides the **fastest onset of action** for opioids, as the medication is directly introduced into the bloodstream. - This route is critically important in **acute pain management**, surgical settings, and emergency situations where immediate pain relief is necessary.

Question 385: All of the following drugs cross the blood-brain barrier, EXCEPT:

A. Physostigmine
B. Atropine
C. Neostigmine (Correct Answer)
D. Lignocaine

Explanation: ***Neostigmine*** - As a **quaternary ammonium compound**, neostigmine carries a permanent positive charge, making it highly **hydrophilic** and unable to readily cross the **blood-brain barrier (BBB)**. - Its inability to cross the BBB means its effects are primarily limited to the **peripheral nervous system**, particularly at the neuromuscular junction for conditions like myasthenia gravis, or in the gut for paralytic ileus. *Physostigmine* - Physostigmine is a **tertiary amine** that is **lipid-soluble** at physiological pH, allowing it to easily cross the **blood-brain barrier** and exert central nervous system effects. - It is used to treat **central anticholinergic syndrome** because it can reverse the central effects of anticholinergic drugs. *Atropine* - Atropine is a **tertiary amine** that is also **lipid-soluble** and readily crosses the **blood-brain barrier**, leading to significant central nervous system effects such as sedation, excitation, or even delirium at higher doses. - Its ability to cross the BBB contributes to its wide range of systemic anticholinergic effects, including those on the brain. *Lignocaine* - Lignocaine (lidocaine) is a **tertiary amine local anesthetic** that is highly **lipid-soluble** and can readily cross the **blood-brain barrier**. - Its entry into the CNS is responsible for its potential side effects such as **seizures, dizziness, and central nervous system depression** when absorbed systemically.

Question 386: After IV drug administration, elimination of a drug depends on:

A. Lipid solubility
B. Volume of distribution
C. Clearance (Correct Answer)
D. All of the options

Explanation: ***Clearance*** - **Clearance (CL)** is the primary and direct determinant of drug elimination after IV administration. - It represents the **volume of plasma cleared of drug per unit time** (e.g., mL/min or L/hr). - The **rate of elimination** is directly calculated as: Rate = CL × Plasma concentration - Clearance integrates the efficiency of all eliminating organs (liver, kidneys) and is the key parameter determining how fast a drug is removed from the body. - Formula: **CL = Rate of elimination / Plasma concentration** *Lipid solubility* - Lipid solubility affects drug **distribution** and **renal reabsorption** but does not directly determine the rate of elimination. - Highly lipid-soluble drugs may be reabsorbed in renal tubules, but the elimination rate is still governed by clearance. - Lipid solubility is more relevant to drug distribution and metabolism pathways than to the rate of elimination itself. *Volume of distribution* - Volume of distribution (Vd) describes how extensively a drug distributes into tissues versus plasma. - While Vd affects the **half-life** (t½ = 0.693 × Vd/CL), it does NOT directly determine the elimination rate. - A large Vd means more drug in tissues, which affects how long elimination takes, but the actual rate of elimination is still determined by clearance. - Vd is a distribution parameter, not an elimination parameter. *All of the options* - This is incorrect because only **clearance** directly determines the rate of drug elimination. - While lipid solubility and volume of distribution can indirectly influence how long a drug remains in the body, they do not determine the elimination rate itself—clearance does.

Question 387: Which of the following is excreted in saliva?

A. Lithium (Correct Answer)
B. Chloramphenicol
C. Ampicillin
D. Tetracycline

Explanation: ***Lithium*** - **Lithium** is actively excreted in saliva, making salivary lithium levels a potential, though not routinely used, indicator of serum levels. - The salivary glands can concentrate lithium, leading to concentrations in saliva that are typically **2 to 3 times higher** than in plasma (saliva/plasma ratio of approximately 2-3:1). - This property makes saliva a useful non-invasive medium for therapeutic drug monitoring of lithium. *Chloramphenicol* - **Chloramphenicol** is primarily metabolized in the liver by glucuronidation and excreted in the urine. - While small amounts may be found in various body fluids, it is not a significant component of salivary excretion. *Ampicillin* - **Ampicillin**, a penicillin antibiotic, is mainly eliminated unchanged via renal excretion. - Salivary excretion is not a primary route of elimination for ampicillin. *Tetracycline* - **Tetracycline** antibiotics are primarily excreted unchanged by the kidneys and, to a lesser extent, in bile. - While some drugs can be detected in saliva, tetracycline is not notably excreted through this route in clinically significant amounts.

Question 388: Which one of the following is true for competitive antagonism?

A. Agonist and competitive antagonist bind to the same receptor (Correct Answer)
B. Agonist cannot displace an antagonist from the receptor
C. Antagonism cannot be completely reversed by an increased dose of an agonist
D. Maximum response (Emax) is reduced in the presence of a competitive antagonist

Explanation: ***Agonist and competitive antagonist bind to the same receptor*** - In **competitive antagonism**, both the **agonist** and the **antagonist** compete for the **same binding site** on the receptor. - This competition means that the effect of the antagonist can be **overcome by increasing the concentration of the agonist** (reversible antagonism). - The binding is **reversible** and depends on the **relative concentrations** and affinities of both molecules. *Agonist cannot displace an antagonist from the receptor* - This is **incorrect** for competitive antagonism; a high concentration of the **agonist** can indeed displace the antagonist from the receptor binding site. - This **reversibility** is a defining characteristic of competitive antagonism. *Antagonism cannot be completely reversed by an increased dose of an agonist* - This is **false** for competitive antagonism; a sufficiently high dose of **agonist** can completely overcome the effect of a competitive antagonist. - This describes **non-competitive** or **irreversible antagonism**, not competitive antagonism. *Maximum response (Emax) is reduced in the presence of a competitive antagonist* - This is **incorrect** for competitive antagonism; the **Emax remains unchanged**. - In competitive antagonism, only the **EC50 increases** (curve shifts right), but the maximum response is still achievable with sufficient agonist. - **Reduced Emax** is characteristic of **non-competitive antagonism**.

Question 389: All of the following are advantages of transdermal drug delivery systems except

A. They minimize interindividual variation in the achieved plasma drug concentration
B. They avoid hepatic first pass metabolism of the drug
C. They produce high peak plasma concentration of the drug (Correct Answer)
D. They produce smooth and nonfluctuating plasma concentration of the drug

Explanation: ***They produce high peak plasma concentration of the drug*** - Transdermal drug delivery generally aims to provide **sustained**, **controlled drug release** over an extended period, leading to relatively **flat plasma concentration profiles**, not high peaks [1]. - High peak concentrations are more characteristic of bolus intravenous injections or immediate-release oral formulations, which can lead to rapid onset but also potentially higher **adverse effects** [1]. *They minimize interindividual variation in the achieved plasma drug concentration* - Transdermal systems often contribute to **consistent drug absorption**, bypassing factors like gastric emptying time, gut motility, and food interactions, which can vary significantly between individuals. - This consistency helps in achieving more **predictable plasma drug levels** compared to oral routes, reducing variability [4]. *They avoid hepatic first pass metabolism of the drug* - Drugs administered transdermally are absorbed directly into the **systemic circulation**, bypassing the **portal venous system** and liver entirely [4]. - This means the drug avoids being metabolized by **hepatic enzymes** before reaching its site of action, increasing its **bioavailability** [4]. *They produce smooth and nonfluctuating plasma concentration of the drug* - Transdermal patches are designed for **continuous drug release** at a controlled rate, maintaining therapeutic levels of the drug in the blood [2], [3]. - This **steady-state drug delivery** helps avoid the "peaks and troughs" seen with intermittent dosing, which can be associated with side effects or suboptimal efficacy [3].

Question 390: Transdermal patch is not used for the following drug?

A. GTN
B. Naloxone (Correct Answer)
C. Fentanyl
D. Nicotine

Explanation: Naloxone - **Naloxone** is an **opioid antagonist** primarily used for the emergency reversal of opioid overdose, requiring a rapid onset of action [3]. - Its therapeutic goal is immediate, high systemic concentrations, which is not suitable for the slow, sustained release characteristic of a transdermal patch. *GTN* - **Glyceryl trinitrate (GTN)** is used in a transdermal patch for the **prophylaxis of angina**, providing a sustained release [1]. - This allows for consistent vasodilation and reduction of cardiac workload over an extended period [1]. *Fentanyl* - **Fentanyl** transdermal patches are commonly used for the management of **chronic severe pain**, particularly in opioid-tolerant patients [2]. - The patch provides continuous systemic delivery of the potent opioid, offering long-lasting pain relief [2]. *Nicotine* - **Nicotine** patches are widely used as **nicotine replacement therapy (NRT)** to aid in smoking cessation. - They deliver a steady dose of nicotine transdermally, reducing withdrawal symptoms and cravings.

Practice by Chapter

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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