Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics Indian Medical PG Practice Questions and MCQs

Question 441: Which of the following is NOT an advantage of sustained release preparations over conventional preparations?

A. Improved compliance
B. Less incidence of high peak side effects
C. Decreased frequency of administration
D. Drugs with very long half-life are suitable (Correct Answer)

Explanation: ***Drugs with very long half-life are suitable*** - Sustained-release preparations are most suitable for drugs with a **half-life between 2 to 8 hours** (some sources suggest 3-12 hours). - Drugs with a **very long half-life (>12-24 hours)** are already long-acting enough that a sustained-release formulation offers **little to no additional benefit**. - Such drugs already maintain therapeutic levels for extended periods without the need for special formulation. - Formulating a drug with a very long half-life into a sustained-release form could lead to **excessive accumulation** and make dose adjustments difficult. - Therefore, suitability of drugs with very long half-life is **NOT an advantage** - it's actually a limitation of sustained-release technology. *Decreased frequency of administration* - This is a primary advantage of sustained release, as it allows for **less frequent dosing intervals** (e.g., once or twice daily instead of multiple times a day). - Reduced dosing frequency helps maintain **therapeutic drug levels** over a longer period without repeated administration. *Improved compliance* - Less frequent dosing directly contributes to **better patient adherence** to the medication regimen. - Patients are more likely to take their medication as prescribed when the **dosing schedule is simpler and less demanding**. *Less incidence of high peak side effects* - Sustained-release formulations provide a **slow and continuous drug release**, avoiding the rapid absorption and high plasma peaks seen with conventional formulations. - This smoother plasma concentration profile leads to **fewer dose-related side effects** associated with high peak drug concentrations.

Question 442: Which of the following statements about oxcarbazepine is incorrect?

A. Metabolises itself
B. Less chances of hepatotoxicity than carbamazepine
C. Less chances of hyponatremia than carbamazepine (Correct Answer)
D. It is less enzyme inducer than carbamazepine

Explanation: ***Correct Answer: Less chances of hyponatremia than carbamazepine*** - This statement is **incorrect** because oxcarbazepine has a **higher incidence of hyponatremia** (up to 25% of patients) compared to carbamazepine (5-10%). - Hyponatremia is a well-known and dose-related adverse effect of oxcarbazepine, often requiring monitoring of serum sodium levels. - This is the most definitively incorrect statement among the options. *Incorrect: Metabolises itself* - While this statement is somewhat ambiguous, it likely refers to **autoinduction of metabolism**, which is a characteristic of **carbamazepine** [1]. - **Oxcarbazepine** is a prodrug that is rapidly converted to its active metabolite monohydroxy derivative (MHD), but it **does not autoinduce its own metabolism** to a significant extent like carbamazepine does [1]. - However, if interpreted literally, oxcarbazepine does undergo metabolism, making this statement potentially confusing. *Incorrect: Less chances of hepatotoxicity than carbamazepine* - This statement is **correct** about oxcarbazepine. It is generally associated with a **lower risk of severe hepatotoxicity** [1] compared to carbamazepine. - While liver enzyme elevations can occur, serious liver damage is much less common with oxcarbazepine. *Incorrect: It is less enzyme inducer than carbamazepine* - This statement is **correct** about oxcarbazepine. Its active metabolite MHD is a **weaker inducer of cytochrome P450 enzymes** (especially CYP3A4) than carbamazepine. - This translates to fewer and less significant drug-drug interactions with oxcarbazepine.

Question 443: Most variable absorption is seen with which route?

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

Explanation: ***Oral*** - **Oral absorption** is highly variable due to numerous factors, including stomach pH, presence of food, drug formulation, and individual patient differences in gastrointestinal motility and enzyme activity [1], [3]. - The drug must first dissolve and then pass through the **intestinal wall** into the bloodstream, where it is also subject to **first-pass metabolism** in the liver, all contributing to variability [2]. *Intramuscular* - While generally reliable, **intramuscular absorption** can vary depending on factors such as muscle blood flow, which can be influenced by activity, temperature, and depth of injection. - However, it bypasses the harsh gastrointestinal environment and has less intrinsic variability than oral administration. *Intravenous* - **Intravenous administration** provides the most direct and complete absorption, as the drug is introduced directly into the bloodstream, bypassing all absorption barriers. - Therefore, it exhibits **no variability in absorption** (bioavailability is 100%), making this option incorrect [4]. *Per rectal* - **Rectal absorption** can be variable, as it depends on factors like the drug's lipid solubility, the pH of the rectal fluid, and the presence of fecal matter. - However, it typically avoids **first-pass metabolism** to some extent and often provides more predictable absorption than oral routes, especially for drugs irritating to the stomach.

Question 444: Which of the following drug substrate combinations is incorrectly matched?

A. CYP 3A4/5 - simvastatin
B. CYP 2D6 - SSRI
C. CYP 2C19 - omeprazole
D. CYP 2C8/9 - mifepristone (Correct Answer)

Explanation: ***CYP 2C8/9 - mifepristone*** - **Mifepristone** is primarily metabolized by **CYP 3A4**, not CYP 2C8/9. - CYP 2C8/9 metabolizes drugs like **warfarin**, rosiglitazone, and tolbutamide. *CYP 3A4/5 - simvastatin* - **Simvastatin** is a known substrate for **CYP 3A4/5**, and its metabolism can be significantly affected by CYP 3A4/5 inhibitors or inducers. - This interaction is clinically important for **drug-drug interactions** concerning statin therapy. *CYP 2D6 - SSRI* - Many **Selective Serotonin Reuptake Inhibitors (SSRIs)**, such as fluoxetine, paroxetine, and venlafaxine, are metabolized by **CYP 2D6**. - This makes CYP 2D6 a crucial enzyme in determining **SSRI efficacy** and **side effect profiles**. *CYP 2C19 - omeprazole* - **Omeprazole**, a proton pump inhibitor, is primarily metabolized by **CYP 2C19** and also by CYP 3A4. - Genetic polymorphisms in **CYP 2C19** can significantly affect omeprazole's metabolism and clinical efficacy.

Question 445: A patient given digoxin started having side effects like nausea and vomiting. The serum concentration of digoxin was 4 ng/mL. The plasma therapeutic range is 1-2 ng/mL. If the half-life of digoxin is 40 hours, how long should one wait before resuming the treatment?

A. 120 hours
B. 140-180 hours
C. 1 half-life (40 hours)
D. 80 hours (Correct Answer)

Explanation: ***80 hours (2 half-lives)***- Current digoxin level is **4 ng/mL**, which is **twice the upper therapeutic limit** (2 ng/mL), causing toxicity with nausea and vomiting [1]- After **1 half-life (40 hours)**: concentration reduces to 2 ng/mL (upper therapeutic limit) [2]- After **2 half-lives (80 hours)**: concentration reduces to 1 ng/mL (mid-therapeutic range) [2]- **Clinical rationale**: While 2 ng/mL is technically within range, waiting for 2 half-lives ensures the level is comfortably in the **middle of the therapeutic window** (1 ng/mL), providing a **safer margin** before resuming treatment in a patient who just experienced toxicity- This conservative approach minimizes risk of recurrent toxicity, especially important given the patient's recent symptoms at 4 ng/mL*1 half-life (40 hours)*- After 1 half-life, digoxin level would be 2 ng/mL, which is at the **upper limit** of the therapeutic range- While technically within the therapeutic range, this leaves **minimal safety margin** in a patient who just experienced toxicity- Starting treatment immediately at this level carries higher risk of recurrent side effects*120 hours (3 half-lives)*- After 3 half-lives, the concentration would be **0.5 ng/mL**, which is **below the therapeutic range** (1-2 ng/mL)- This is overly conservative and would **unnecessarily delay** resumption of essential cardiac medication- Could lead to inadequate control of the underlying condition (heart failure or atrial fibrillation)*140-180 hours (3.5-4.5 half-lives)*- This would reduce digoxin to **0.25-0.35 ng/mL**, well below therapeutic levels- This **excessive delay** is not clinically justified and could worsen the patient's cardiac condition- No standard protocol recommends waiting this long before resuming digoxin therapy

Question 446: Pegloticase reduces serum uric acid levels by which mechanism?

A. Converts uric acid to allantoin (Correct Answer)
B. Inhibits xanthine oxidase
C. Inhibits uric acid reabsorption in the kidneys
D. Increases renal excretion of uric acid

Explanation: ***Converts uric acid to allantoin*** - Pegloticase is a **recombinant uricase enzyme** that catalyzes the oxidation of uric acid to **allantoin**. - Allantoin is a **water-soluble** substance that is easily excreted by the kidneys, thereby lowering serum uric acid levels. *Inhibits xanthine oxidase* - This is the mechanism of action for drugs like allopurinol and febuxostat, which prevent the formation of uric acid. - Pegloticase works downstream of uric acid formation, by enhancing its degradation. *Inhibits uric acid reabsorption in the kidneys* - This is the mechanism of action for **uricosuric agents** such as probenecid and lesinurad. - These drugs increase the excretion of uric acid by acting on transporters in the renal tubules, but this is not how pegloticase works. *Increases renal excretion of uric acid* - While pegloticase ultimately leads to increased renal excretion of allantoin, its primary mechanism is the **conversion of uric acid to allantoin**, not direct action on renal tubules to excrete uric acid. - Uricosuric agents directly increase renal excretion of uric acid.

Question 447: Which of the following statements is true about noncompetitive enzyme inhibitors?

A. Noncompetitive inhibitors increase Km and decrease Vmax
B. Km remains same, Vmax decreases (Correct Answer)
C. Km decreases, Vmax increases
D. Km increases, Vmax increases

Explanation: ***Km remains same, Vmax decreases*** - **Noncompetitive inhibitors** bind to an allosteric site on the enzyme, altering its conformation and **reducing its catalytic efficiency** without affecting substrate binding affinity. - This results in a decreased **Vmax** (maximum reaction rate) because the enzyme's ability to process substrate is impaired, while the **Km** (substrate concentration at half Vmax) remains unchanged as the substrate can still bind efficiently to the active site. *Noncompetitive inhibitors increase Km and decrease Vmax* - This statement is incorrect because noncompetitive inhibitors primarily affect the enzyme's **catalytic activity** (Vmax) rather than its affinity for the substrate (Km). - An increase in **Km** would imply a decrease in the enzyme's affinity for the substrate, which is characteristic of **competitive inhibition**, not noncompetitive inhibition. *Km decreases, Vmax increases* - This statement describes an effect not typically seen with noncompetitive inhibition; a decreased **Km** would imply increased substrate affinity, and an increased **Vmax** would suggest enhanced catalytic efficiency. - Noncompetitive inhibitors are known to **reduce** the maximum reaction rate, not increase it. *Km increases, Vmax increases* - This scenario, where both **Km** and **Vmax** increase, is not characteristic of any standard type of enzyme inhibition. - An increase in **Vmax** implies an enhancement in enzyme activity, which is contrary to the action of an inhibitor.

Question 448: At pKa = pH, what is the relationship between the ionic and non-ionic forms of a drug?

A. Absorption of drug is 50% ionic and 50% non-ionic
B. Conc. of drug is 25% ionic and 75% non-ionic
C. Conc. of drug is 75% ionic and 25% non-ionic
D. Conc. of drug is 50% ionic and 50% non-ionic (Correct Answer)

Explanation: ***Conc. of drug is 50% ionic and 50% non-ionic*** - At **pKa = pH**, the concentrations of the **ionized** and **unionized** forms of a drug are equal as per the **Henderson-Hasselbalch equation**. - This means that exactly **half** of the drug molecules are in their charged (ionic) state, and the other half are in their uncharged (non-ionic) state. *Absorption of drug is 50% ionic and 50% non-ionic* - The amount of drug that is absorbed is dependent on the **non-ionic concentration** available at the absorption site, but this option incorrectly states that the *absorption itself* is 50% ionic. - Absorption primarily occurs for the **non-ionic, lipophilic form** as it can more readily cross cell membranes. *Conc. of drug is 75% ionic and 25% non-ionic* - This ratio would occur when the **pH** is either 0.5 units above the pKa for a weak acid or 0.5 units below the pKa for a weak base. - For example, if **pH = pKa + 0.5** (for a weak acid), approximately 75% would be ionic. *Conc. of drug is 25% ionic and 75% non-ionic* - This ratio would occur when the **pH** is either 0.5 units below the pKa for a weak acid or 0.5 units above the pKa for a weak base. - For example, if **pH = pKa - 0.5** (for a weak acid), approximately 25% would be ionic.

Question 449: Variation in sensitivity of response to different doses of a drug in different individuals is obtained from?

A. Dose-response relationship (Correct Answer)
B. Therapeutic index
C. Bioavailability
D. Phase 1 clinical trials

Explanation: ***Dose-response relationship*** - The **dose-response relationship** (particularly the **graded dose-response curve**) describes how the magnitude of a drug's effect changes with different doses. - When plotted for different individuals or populations, these curves reveal **variation in sensitivity** through differences in potency (horizontal shift) and efficacy (maximum response). - This relationship helps characterize inter-individual variability in drug response and is the fundamental concept for understanding differential sensitivity. *Therapeutic index* - The **therapeutic index** is a measure of drug safety, representing the ratio between the toxic dose and the effective dose (TD50/ED50 or LD50/ED50). - It does not directly explain the variation in sensitivity to different doses among individuals, but rather provides information about the drug's overall safety margin. *Bioavailability* - **Bioavailability** refers to the fraction of an administered drug that reaches the systemic circulation unchanged. - While it influences the drug concentration at the site of action, it doesn't directly measure the variability in physiological response to that concentration among individuals. *Phase 1 clinical trials* - **Phase 1 clinical trials** are the first stage of testing a new drug in humans, primarily focusing on safety, dosage range, and pharmacokinetics in a small group of healthy volunteers. - While variability in response may be observed during these trials, they are not the *pharmacological concept* that describes this variation; rather, dose-response relationships are used to interpret findings from these trials.

Question 450: Imipenem, a newer antibiotic with a broad antibacterial spectrum, is co-administered with cilastatin. What is the primary reason for this combination?

A. Cilastatin enhances the gastrointestinal absorption of imipenem.
B. Cilastatin inhibits the beta-lactamase enzyme that degrades imipenem.
C. Cilastatin prevents the degradation of imipenem by inhibiting an enzyme in the kidneys. (Correct Answer)
D. The combination of antibiotics is synergistic against Pseudomonas species.

Explanation: ***Cilastatin prevents the degradation of imipenem by inhibiting an enzyme in the kidneys.*** - **Imipenem** is susceptible to degradation by **dehydropeptidase-1 (DHP-1)**, an enzyme found in the renal tubules, leading to inactivation and the production of potentially nephrotoxic metabolites. - **Cilastatin** is a **DHP-1 inhibitor** that prevents the inactivation of imipenem in the kidneys, thereby increasing its urinary concentration and reducing the risk of renal toxicity. *Cilastatin enhances the gastrointestinal absorption of imipenem.* - **Imipenem** is administered parenterally (intravenously) because it has **poor oral bioavailability** and is extensively metabolized in the gastrointestinal tract. - Cilastatin’s primary role is not to enhance GI absorption, as the drug is not intended for oral administration. *Cilastatin inhibits the beta-lactamase enzyme that degrades imipenem.* - **Imipenem** itself is highly resistant to most bacterial **beta-lactamase enzymes** due to its unique carbapenem structure. - Cilastatin does not have significant beta-lactamase inhibitory activity; its main function is renal enzyme inhibition. *The combination of antibiotics is synergistic against Pseudomonas species.* - While both imipenem and cilastatin together are effective due to preservation of imipenem, the combination itself is not a synergistic pair of antibiotics in the traditional sense against *Pseudomonas*. - Synergy typically refers to two different antibiotics working together, whereas cilastatin is an enzyme inhibitor, not an antibiotic.

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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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