Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics Indian Medical PG Practice Questions and MCQs

Question 401: Which of the following cytochromes is involved in monooxygenase mediated detoxification of drugs?

A. Cytochrome b5
B. Cytochrome P450 (Correct Answer)
C. Cytochrome c
D. NADPH-cytochrome P450 reductase

Explanation: ***Cyt P 450*** - **Cytochrome P450** enzymes are a superfamily of **monooxygenases** that play a critical role in the metabolism and detoxification of a wide variety of endogenous and exogenous substances, including drugs. - They facilitate phase I reactions (e.g., **oxidation**, reduction, hydrolysis), which typically introduce or expose functional groups to make compounds more polar and easier to excrete. *Cytochrome b5* - **Cytochrome b5** is involved in various metabolic reactions, including **fatty acid desaturation** and cholesterol biosynthesis, and can sometimes interact with P450 systems but is not the primary monooxygenase for drug detoxification. - It also participates in the reduction of methemoglobin and can act as an electron donor, but its role in drug detoxification is secondary and accessory to P450. *Cytochrome c* - **Cytochrome c** is a key component of the **electron transport chain** in mitochondria, primarily involved in cellular respiration and ATP production. - It has a crucial role in **apoptosis** when released into the cytosol, but it is not directly involved in drug monooxygenase detoxification. *NADPH-cytochrome P450 reductase* - **NADPH-cytochrome P450 reductase** is an enzyme that transfers electrons from NADPH to **cytochrome P450 enzymes**, enabling their monooxygenase activity. - While essential for P450 function, it is the **reductase** (electron donor) and not the monooxygenase enzyme itself, which is Cytochrome P450.

Question 402: Which of the following opioid antagonists can be administered orally for long-term therapy?

A. Buprenorphine
B. Nalbuphine
C. Naloxone
D. Naltrexone (Correct Answer)

Explanation: ***Naltrexone*** - **Naltrexone** is an opioid antagonist with good oral bioavailability, making it suitable for **long-term management** of opioid or alcohol dependence. - Its long duration of action (up to 24 hours) allows for **once-daily dosing**, which improves patient adherence for chronic conditions. - It competitively blocks opioid receptors (primarily μ-receptors) and is FDA-approved for maintenance therapy in opioid use disorder. *Buprenorphine* - **Buprenorphine** is a **partial μ-opioid agonist** and κ-opioid antagonist (mixed agonist-antagonist). - While it can be used in opioid dependence treatment, it acts primarily as a **partial agonist** providing some opioid effects, not as a pure antagonist. - It is used for opioid substitution therapy and pain management, not for complete opioid receptor blockade. *Nalbuphine* - **Nalbuphine** is a **mixed agonist-antagonist** (κ-agonist and μ-antagonist) used primarily for moderate to severe pain. - It has limited use due to its ceiling effect on analgesia and potential for dysphoria at high doses. - It is not suitable for long-term oral therapy in opioid dependence management. *Naloxone* - **Naloxone** is a pure opioid antagonist used for **acute opioid overdose** reversal. - It has rapid onset but a **short half-life (30-90 minutes)**, making it impractical for long-term therapy. - Poor oral bioavailability (<3% due to extensive first-pass metabolism) makes it unsuitable for oral administration in maintenance therapy.

Question 403: Which of the following does not penetrate intact skin?

A. Nitroglycerine
B. Clonidine
C. Dexmedetomidine (Correct Answer)
D. Scopolamine

Explanation: ***Dexmedetomidine*** - **Dexmedetomidine** is primarily administered intravenously as a continuous infusion for sedation and analgesia, as its molecular structure and properties do not allow for significant transdermal absorption. - While some research explores its transdermal delivery, it is **not considered to penetrate intact skin effectively** for therapeutic use through this route in current clinical practice. *Nitroglycerin* - **Nitroglycerin** patches and ointments are commonly used for the transdermal delivery of the drug to treat anginal symptoms. - Its **lipophilic nature** and small molecular size allow it to readily penetrate the skin barrier and enter systemic circulation. *Clonidine* - **Clonidine** is available in transdermal patch formulations for the treatment of hypertension. - Its ability to penetrate intact skin allows for **sustained release** and systemic absorption, providing continuous blood pressure control. *Scopolamine* - **Scopolamine** is widely used in transdermal patches to prevent motion sickness. - Its **small molecular weight** and lipophilicity enable effective penetration through the skin.

Question 404: Longest acting local anesthetic agent is -

A. Bupivacaine (Correct Answer)
B. Procaine
C. Lidocaine
D. Dibucaine

Explanation: ***Bupivacaine*** - **Bupivacaine** is the **longest-acting local anesthetic** in common clinical use, with a duration of action of **2-9 hours** (up to 12+ hours with epinephrine). - Its prolonged effect is due to **high lipid solubility** and **extensive protein binding** (95%), allowing it to remain at the nerve site for an extended period. - Widely used for **epidural anesthesia**, **spinal anesthesia**, and **peripheral nerve blocks** requiring prolonged analgesia. *Dibucaine* - Dibucaine, while theoretically long-acting, is **rarely used clinically** in modern practice. - Primarily known as a **research tool** for testing plasma pseudocholinesterase activity (dibucaine number test). - Not a standard answer for competitive medical examinations. *Procaine* - **Procaine** is a **short-acting** local anesthetic (30-60 minutes), primarily used for infiltration. - Its rapid metabolism by **plasma pseudocholinesterase** limits its duration of action. *Lidocaine* - **Lidocaine** is an **intermediate-acting** local anesthetic (1-3 hours), widely used for various procedures due to its rapid onset and moderate duration. - Its duration is significantly shorter than bupivacaine.

Question 405: Low apparent volume of distribution of drug indicates that:

A. Drug has low bioavailability
B. Drug has low efficacy
C. Drug is not extensively distributed to tissue (Correct Answer)
D. Drug has low half life

Explanation: ***Drug is not extensively distributed to tissue*** - A **low apparent volume of distribution (Vd)** suggests that the drug primarily remains in the **vascular compartment**. - This indicates **minimal binding to peripheral tissues** and less distribution into extravascular spaces. *Drug has low bioavailability* - **Bioavailability** refers to the fraction of an administered drug that reaches the **systemic circulation unchanged**. - While related to drug disposition, a low Vd does not directly imply low bioavailability; a drug can have high bioavailability but remain largely in the blood. *Drug has low efficacy* - **Efficacy** is the maximum effect a drug can produce regardless of the dose. - Vd relates to drug distribution, not its pharmacological effect or **intrinsic activity** at its target. *Drug has low half life* - The **half-life** of a drug is determined by its **volume of distribution (Vd)** and **clearance (CL)** (t½ = 0.693 × Vd / CL). - While a low Vd can contribute to a shorter half-life if clearance is high, Vd alone does not solely determine half-life; clearance also plays a significant role.

Question 406: Duration of action of proparacaine:

A. 2min
B. 20min (Correct Answer)
C. 5min
D. 10min

Explanation: ***20min*** - **Proparacaine** is a **topical ophthalmic anesthetic** widely used for procedures like **tonometry**, **foreign body removal**, and **minor ocular surgeries**. - Its **duration of action** is typically **15-20 minutes**, making **20 minutes** the most accurate answer representing the standard effective duration. - This duration provides adequate anesthesia for most diagnostic and minor procedural needs in ophthalmology. *10min* - While proparacaine may provide some anesthetic effect at **10 minutes**, this represents the **lower end** of its duration and is **not the standard cited duration** in medical literature. - Most authoritative sources cite the duration as **15-20 minutes** rather than 10 minutes. - Relying on only 10 minutes of action would be insufficient for many ophthalmic procedures. *2min* - A duration of **2 minutes** is far too short for proparacaine's clinical applications. - Although proparacaine has a **rapid onset** (20-30 seconds), its anesthetic effect persists much longer. - This duration would be inadequate for even the briefest procedures. *5min* - **5 minutes** significantly underestimates proparacaine's duration of action. - This would not provide sufficient anesthesia for most diagnostic or therapeutic ophthalmic procedures. - Clinical practice and pharmacological data support a much longer duration.

Question 407: All of the following drugs require dose reduction in renal failure except?

A. Doxycycline (Correct Answer)
B. Vancomycin
C. Gentamicin
D. Acyclovir

Explanation: ***Doxycycline*** - **Doxycycline** is primarily eliminated via the gastrointestinal tract (fecal excretion) and does NOT require dose adjustment in patients with **renal impairment**. [1] - Its unique elimination pathway makes it a safe choice for treating infections in patients with **renal failure**. - This is a key distinguishing feature among tetracyclines. *Vancomycin* - **Vancomycin** is predominantly eliminated by the kidneys (80-90% unchanged in urine). - Accumulation in renal failure can lead to **ototoxicity** and **nephrotoxicity**. - Dosage must be carefully adjusted based on **creatinine clearance** and therapeutic drug monitoring is essential. *Gentamicin* - **Gentamicin**, an aminoglycoside, is almost entirely excreted unchanged by the kidneys. - Highly **nephrotoxic** and **ototoxic** with narrow therapeutic index. - Dose reduction and extended dosing intervals are critical in **renal failure** to prevent drug accumulation and serious adverse effects. [3] *Acyclovir* - **Acyclovir** is primarily eliminated renally (60-90% excreted unchanged in urine). - Requires significant **dose reduction in renal impairment** to prevent crystalluria and neurotoxicity. [2] - Dosing adjustment based on creatinine clearance is mandatory to avoid adverse effects.

Question 408: All are true for cytochrome P450 enzymes EXCEPT:

A. Synthesize amino acids (Correct Answer)
B. Involved in drug metabolism
C. Present mainly in the liver
D. Part of Phase I metabolism

Explanation: ***Synthesize amino acids*** - Cytochrome P450 enzymes are primarily involved in the **metabolism of xenobiotics** and endogenous compounds, not in the synthesis of amino acids. - **Amino acid synthesis** occurs through different metabolic pathways involving various enzymes distinct from the cytochrome P450 system. *Involved in drug metabolism* - Cytochrome P450 enzymes are a major group of enzymes crucial for the **biotransformation of numerous drugs** and other foreign compounds. - They typically catalyze **oxidation reactions**, preparing drugs for excretion. *Present mainly in the liver* - While present in many tissues, the **highest concentration and diversity** of cytochrome P450 enzymes are found in the **liver**, which is the primary site of drug metabolism. - They are also found in the gastrointestinal tract, kidney, lung, and brain, but to a lesser extent. *Part of Phase I metabolism* - Cytochrome P450 enzymes are the **principal enzymes responsible for Phase I reactions** in drug metabolism. - **Phase I metabolism** generally involves reduction, oxidation, or hydrolysis reactions to introduce polar groups to the drug molecule.

Question 409: What is the primary role of Cytochrome P450 enzymes in the liver?

A. Lipid transport
B. Oxidation of drugs (Correct Answer)
C. Carbohydrate synthesis
D. Protein degradation

Explanation: ***Oxidation of drugs*** - **Cytochrome P450 enzymes** are a superfamily of monooxygenases that primarily catalyze the **oxidation of various endogenous and exogenous substrates**, including drugs [1, 2]. - This oxidative metabolism is a key step in detoxification and elimination of foreign compounds from the body [1]. *Lipid transport* - **Lipid transport** is primarily facilitated by **lipoproteins** and specific **transport proteins** in the blood and within cells. - While P450 enzymes can metabolize some lipids, their primary role is not in lipid transport [2]. *Carbohydrate synthesis* - **Carbohydrate synthesis**, or **gluconeogenesis**, is mainly carried out by enzymes such as **pyruvate carboxylase** and **fructose-1,6-bisphosphatase**. - Cytochrome P450 enzymes do not play a direct role in the synthesis of carbohydrates. *Protein degradation* - **Protein degradation** is largely mediated by the **ubiquitin-proteasome system** and **lysosomal pathways**. - Cytochrome P450 enzymes are not directly involved in breaking down proteins into smaller peptides or amino acids.

Question 410: Acetazolamide is given to a patient with angle closure glaucoma. It is a reversible competitive inhibitor of carbonic anhydrase enzyme. Which of the following should be the effect of this drug?

A. No change in Vmax (Correct Answer)
B. Increase in both Km and Vmax
C. Decrease in Km
D. Decrease in Vmax

Explanation: ***No change in Vmax*** - **Competitive inhibitors** bind reversibly to the active site of the enzyme, competing with the substrate for binding [1]. - At sufficiently high substrate concentrations, the substrate can outcompete the inhibitor, allowing the enzyme to reach its **maximum velocity (Vmax)**. - Therefore, Vmax remains unchanged in competitive inhibition, though more substrate is needed to achieve it [2]. *Decrease in Vmax* - A decrease in Vmax is characteristic of **non-competitive inhibitors**, which bind to a site other than the active site and reduce the enzyme's catalytic efficiency. - In competitive inhibition, Vmax is not decreased because high substrate concentrations can overcome the inhibition [2]. *Increase in both Km and Vmax* - While competitive inhibition does **increase Km** (apparent Km increases because more substrate is needed to reach half-maximal velocity), **Vmax remains unchanged**, not increased. - An increase in Vmax would indicate enhanced enzyme activity, which does not occur with inhibitors. *Decrease in Km* - A **decrease in Km** indicates higher enzyme affinity for substrate, meaning less substrate is needed to reach half-maximal velocity. - Competitive inhibition actually **increases Km** (decreases apparent affinity) because the inhibitor competes with substrate for the active site [1]. *Clinical Application* - Acetazolamide is used preoperatively in acute angle-closure glaucoma to lower intraocular pressure [3].

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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