Pharmacokinetics and Pharmacodynamics Practice Questions

Q: Which of the following drugs has a covalent interaction with its target?

Aspirin. **Explanation:** The correct answer is **Aspirin**. This question tests the concept of **drug-receptor bond types**, specifically irreversible covalent bonding. **1. Why Aspirin is correct:** Aspirin (Acetylsalicylic acid) acts by **irreversibly acetylating** a serine residue in the active site of the **Cyclooxygenase (COX-1 and COX-2)** enzymes. Because a covalent bond is formed, the enzyme is permanently inactivated. In platelets, which lack a nucleus to synthesize new proteins, this effect lasts for the entire lifespan of the platelet (7–10 days), explaining its potent anti-thrombotic effect. **2. Analysis of Incorrect Options:** * **B. Penicillin:** While Penicillin does bind to Penicillin-Binding Proteins (PBPs), it is often categorized under "suicide inhibition" in biochemistry. However, in standard pharmacological classification for NEET-PG, Aspirin is the classic prototype for covalent modification. (Note: Some texts consider Penicillin covalent, but Aspirin remains the most definitive answer in this MCQ context). * **C. Nitric Oxide:** This is a gas that acts as a signaling molecule. It binds **reversibly** to the heme group of soluble guanylyl cyclase to increase cGMP. * **D. Bosentan:** This is a competitive (reversible) antagonist at endothelin (ET-A and ET-B) receptors used in pulmonary hypertension. **High-Yield Clinical Pearls for NEET-PG:** * **Other Covalent/Irreversible Inhibitors:** Omeprazole (H+/K+ ATPase), Organophosphates (AChE), Phenoxybenzamine (Alpha receptors), and Clopidogrel (P2Y12 receptor). * **Aspirin Kinetics:** At low doses, it follows first-order kinetics; at toxic/high doses, it shifts to **zero-order kinetics** (saturation kinetics). * **Key Distinction:** Most drugs form weak, reversible bonds (Ionic, Hydrogen, Van der Waals). Covalent bonds are the strongest and require the synthesis of new receptors/enzymes to overcome the drug effect.

Q: What percentage of a substance is eliminated after 3 half-lives in first-order kinetics?

87.50%. ### Explanation In **first-order kinetics**, a constant **fraction** (percentage) of a drug is eliminated per unit of time. This means the half-life ($t_{1/2}$) remains constant regardless of the plasma concentration. **Step-by-Step Calculation:** To determine the percentage eliminated, we first calculate the amount **remaining** after each half-life: * **After 1st half-life:** 50% remains (50% eliminated). * **After 2nd half-life:** 50% of 50% = 25% remains (75% eliminated). * **After 3rd half-life:** 50% of 25% = **12.5% remains**. To find the amount eliminated: $100\% - 12.5\% = \mathbf{87.5\%}$. --- ### Analysis of Options: * **Option A (12.50%):** This represents the amount of substance **remaining** in the body after 3 half-lives, not the amount eliminated. * **Option B (75%):** This is the percentage eliminated after **2 half-lives**. * **Option D (94%):** This is the approximate percentage eliminated after **4 half-lives** ($100 - 6.25 = 93.75\%$). --- ### High-Yield NEET-PG Pearls: 1. **Steady State:** It takes approximately **4 to 5 half-lives** to reach steady-state concentration ($C_{ss}$) and the same amount of time to completely eliminate a drug from the body. 2. **First-order vs. Zero-order:** Most drugs follow first-order kinetics. In **zero-order kinetics** (e.g., Ethanol, Phenytoin, Aspirin at high doses), a constant **amount** is eliminated, and the half-life is not constant. 3. **Formula:** Percentage eliminated = $100 - (100/2^n)$, where $n$ is the number of half-lives. For $n=3$: $100 - (100/8) = 87.5\%$.

Q: All of the following drugs undergo hepatic metabolism before excretion EXCEPT:

Penicillin G. ### Explanation The correct answer is **Penicillin G**. **1. Why Penicillin G is the correct answer:** Most drugs are lipophilic and require hepatic metabolism (Phase I and Phase II reactions) to become water-soluble for excretion. However, **Penicillin G** is a highly polar, organic acid that is already water-soluble. It is excreted **unchanged** primarily via the kidneys. Specifically, about 90% is excreted through **active tubular secretion** via the Organic Anion Transporter (OAT), and 10% via glomerular filtration. This is why Probenecid (which inhibits OAT) is used clinically to prolong the half-life of penicillin. **2. Why the other options are incorrect:** * **Phenytoin:** This is a classic example of a drug that undergoes extensive hepatic metabolism via CYP2C9. It follows **zero-order kinetics** (saturable metabolism) at high therapeutic doses, a high-yield fact for NEET-PG. * **Diazepam:** A long-acting benzodiazepine that undergoes hepatic oxidation (Phase I) to form active metabolites like desmethyldiazepam, which further increases its duration of action. * **Cimetidine:** While a portion is excreted unchanged, it undergoes significant hepatic metabolism. It is clinically significant as a potent **inhibitor of Cytochrome P450 enzymes**, leading to numerous drug-drug interactions. **3. NEET-PG Clinical Pearls:** * **Renal Excretion Rule:** Drugs excreted primarily unchanged in urine (like Penicillin, Aminoglycosides, Digoxin, and Lithium) require strict **dose adjustment in renal failure**. * **Probenecid Interaction:** Probenecid competes with Penicillin for the tubular secretory pathway, increasing Penicillin plasma levels. * **Zero-Order Kinetics Mnemonic:** Remember "**WATT P**" (Warfarin/Whiskey, Aspirin, Tolbutamide, Theophylline, Phenytoin) for drugs that saturate metabolic pathways.

Q: Which of the following statements about non-competitive antagonism is FALSE?

Efficacy remains the same.. In **non-competitive antagonism**, the antagonist binds to an allosteric site (different from the agonist’s binding site) [2] or binds irreversibly to the active site [1]. This prevents the agonist from producing a maximal effect, regardless of how much agonist concentration is increased [1]. In non-competitive antagonism, the **efficacy (Emax) decreases** [1]. Because the antagonist effectively "removes" functional receptors from the system or prevents the agonist from activating them, the maximum possible response of the drug is reduced [1]. Therefore, the statement "Efficacy remains the same" is false. As the maximal response (Emax) decreases, the height of the log dose-response curve (LDRC) drops, leading to a characteristic "flattening" of the curve [1]. While not all non-competitive antagonists are irreversible, most irreversible antagonists (like Phenoxybenzamine) function non-competitively because they cannot be displaced by increasing agonist concentrations [1]. In pure non-competitive antagonism, the **EC50 (potency)** typically remains unchanged because the remaining functional receptors still have the same affinity for the agonist [1].

Q: A steady state plasma concentration is achieved in how many hours if the half-life of a drug is 40 hours?

180 hours. ### Explanation **1. Understanding Steady State Concentration (Css)** Steady state is reached when the rate of drug administration equals the rate of drug elimination. In clinical pharmacokinetics, it is a standard rule that it takes approximately **4 to 5 half-lives ($t_{1/2}$)** to reach steady state [1]. * After 1 $t_{1/2}$: 50% of Css is reached. * After 2 $t_{1/2}$: 75% of Css is reached. * After 3 $t_{1/2}$: 87.5% of Css is reached. * After 4 $t_{1/2}$: 93.75% of Css is reached [2]. * **After 4.5 to 5 $t_{1/2}$:** >95% of Css is reached (clinically considered steady state) [1]. **Calculation:** Given $t_{1/2} = 40$ hours. Steady state $\approx 4.5 \times 40 = \mathbf{180 \text{ hours}}$. **2. Analysis of Incorrect Options** * **Option A (140 hours):** This represents only 3.5 half-lives. At this point, the drug has not yet reached the therapeutic plateau. * **Option B (160 hours):** This represents exactly 4 half-lives. While close, 4.5 to 5 half-lives is the more accurate physiological benchmark for complete steady state in competitive exams. * **Option D (200 hours):** This represents 5 half-lives. While 5 half-lives is also a valid point for steady state, in many standardized questions (including this one), the calculation based on 4.5 half-lives (180 hours) is the preferred specific answer choice. **3. NEET-PG High-Yield Pearls** * **Independence from Dose:** The time to reach steady state depends **only** on the half-life, not on the dose or the frequency of administration. * **Loading Dose:** To achieve therapeutic levels rapidly without waiting for 4-5 half-lives, a **Loading Dose** is administered. * **Elimination:** Similarly, it takes 4-5 half-lives for a drug to be completely eliminated from the body after stopping the infusion [1]. * **First-Order Kinetics:** This rule applies only to drugs following first-order kinetics (where a constant fraction of drug is eliminated per unit time) [1].

Q: Which of the following parameters signifies the effective drug removal from the body?

Clearance. **Explanation:** **Clearance (CL)** is the most important parameter to consider when determining the efficiency of drug elimination. It is defined as the volume of plasma from which a drug is completely removed per unit of time (e.g., mL/min). Unlike the half-life, which describes the time taken for concentration to reduce by half, clearance directly signifies the body's ability to rid itself of a drug through organs like the kidneys and liver. **Analysis of Incorrect Options:** * **Bioavailability (B):** This refers to the fraction of an administered dose of unchanged drug that reaches the systemic circulation. It measures absorption and first-pass metabolism, not removal. * **Safety (C):** This is a clinical assessment of the risk-to-benefit ratio of a drug. While influenced by clearance (to avoid toxicity), it is not a pharmacokinetic parameter of drug removal. * **Volume of Distribution (Vd):** This relates the amount of drug in the body to the concentration of drug in the plasma. A high Vd indicates the drug is sequestered in tissues, which actually makes it harder to remove from the body via hemodialysis or natural clearance. **High-Yield Clinical Pearls for NEET-PG:** * **Formula:** $CL = Rate\ of\ elimination / Plasma\ concentration$. * **Steady State:** Clearance is the primary determinant of the **maintenance dose** required to achieve a target steady-state plasma concentration. * **First-order Kinetics:** In most drugs, clearance remains constant regardless of the plasma concentration. * **Zero-order Kinetics:** Clearance decreases as plasma concentration increases (e.g., Phenytoin, Alcohol, Aspirin), leading to a high risk of toxicity.

Q: Which of the following drugs is NOT metabolized by the liver?

Penicillin G. ### Explanation **Correct Option: A. Penicillin G** The primary mechanism for the elimination of **Penicillin G** is **renal excretion**, not hepatic metabolism. Approximately 90% of the drug is excreted via active tubular secretion and 10% via glomerular filtration. Because it is excreted unchanged in the urine, its half-life is significantly prolonged in patients with renal failure, necessitating dose adjustments. **Analysis of Incorrect Options:** * **B. Phenytoin:** This is a classic example of a drug metabolized by the liver via the **CYP2C9** and **CYP2C19** enzymes. It follows **zero-order kinetics** (capacity-limited metabolism) at therapeutic or high concentrations, making it a high-yield topic for exams. * **C. Erythromycin:** This macrolide antibiotic is primarily metabolized by the liver (**CYP3A4**) and excreted mainly in the bile. It is also a potent **enzyme inhibitor**, leading to numerous drug-drug interactions. * **D. Cimetidine:** An H2-receptor antagonist that undergoes significant hepatic metabolism. It is well-known in pharmacology as a broad-spectrum **cytochrome P450 inhibitor**. **High-Yield Clinical Pearls for NEET-PG:** * **Probenecid Interaction:** Probenecid inhibits the renal tubular secretion of Penicillin G, thereby increasing its plasma concentration and duration of action. * **Drugs Excreted Unchanged in Urine:** Remember the mnemonic **"GAL"** or **"KEEPS"** (Kanamycin, Ethambutol, Enalapril, Penicillin, Streptomycin). * **Hepatic vs. Renal:** Most lipid-soluble drugs require hepatic metabolism to become polar for excretion, whereas highly polar/water-soluble drugs (like Penicillins and Aminoglycosides) are often excreted unchanged by the kidneys.

Q: A drug has 40% absorption and a hepatic extraction ratio of 0.6. What is the bioavailability of this drug?

16%. ### Explanation **1. Understanding the Concept (Why A is Correct)** Bioavailability ($F$) is the fraction of an administered dose of unchanged drug that reaches the systemic circulation. When a drug is given orally, it must first be absorbed across the gut wall and then pass through the liver (first-pass metabolism) before reaching the systemic blood flow. The formula for bioavailability is: **$F = f \times (1 - ER)$** * **$f$ (Absorption):** The fraction of the dose absorbed from the gut (40% or 0.4). * **$ER$ (Extraction Ratio):** The fraction of the drug removed by the liver during its first pass. * **$(1 - ER)$:** The fraction that escapes hepatic metabolism (Hepatic Bioavailability). **Calculation:** * Absorption ($f$) = 0.4 * Extraction Ratio ($ER$) = 0.6 * Fraction escaping the liver = $1 - 0.6 = 0.4$ * **$F = 0.4 \times 0.4 = 0.16$ or 16%** **2. Analysis of Incorrect Options** * **B (24%):** This is the result of multiplying the absorption (0.4) by the extraction ratio (0.6). This represents the fraction of the dose metabolized by the liver, not the fraction that reaches the circulation. * **C (20%):** This is a distractor often chosen by students who incorrectly average the two numbers or use incorrect ratios. * **D (28%):** This value does not correlate with any standard pharmacokinetic calculation using the provided figures. **3. High-Yield Clinical Pearls for NEET-PG** * **IV Route:** By definition, the bioavailability of a drug administered intravenously is **100% ($F=1$)**. * **First-Pass Effect:** Drugs with a high hepatic extraction ratio (e.g., **Propranolol, Nitroglycerin, Lidocaine, Morphine**) have low oral bioavailability and require significantly higher oral doses compared to IV doses. * **Clinical Significance:** If a patient has liver cirrhosis, the extraction ratio decreases, leading to a potential increase in the bioavailability of high-ER drugs, necessitating dose reduction to avoid toxicity.

Question 1

Phenytoin follows zero-order kinetics at high doses and first-order kinetics at low doses. If 400 mg of phenytoin is present in the body and its rate of elimination at this dose is 200 mg/hr, how much will remain in plasma after 2 hours?

Accepted Answer

Cannot be predicted from this information

Answer

200 mg

Answer

100 mg

Answer

0 mg

Question 2

Which of the following drugs has a covalent interaction with its target?

Accepted Answer

Aspirin

Answer

Penicillin

Answer

Nitric oxide

Answer

Bosentan

Question 3

What percentage of a substance is eliminated after 3 half-lives in first-order kinetics?

Accepted Answer

87.50%

Answer

12.50%

Answer

75%

Answer

94%

Question 4

All of the following drugs undergo hepatic metabolism before excretion EXCEPT:

Accepted Answer

Penicillin G

Answer

Phenytoin

Answer

Diazepam

Answer

Cimetidine

Question 5

Which of the following statements about non-competitive antagonism is FALSE?

Accepted Answer

Efficacy remains the same.

Answer

The dose-response curve flattens.

Answer

The antagonism is irreversible.

Answer

Potency remains the same.

Question 6

A steady state plasma concentration is achieved in how many hours if the half-life of a drug is 40 hours?

Accepted Answer

180 hours

Answer

140 hours

Answer

160 hours

Answer

200 hours

Question 7

Which of the following parameters signifies the effective drug removal from the body?

Accepted Answer

Clearance

Answer

Bioavailability

Answer

Safety

Answer

Volume of distribution

Question 8

Which of the following drugs exhibits zero-order kinetics?

Accepted Answer

Fomepizole

Answer

Phenytoin

Answer

Tolbutamide

Answer

Alcohol

Question 9

Which of the following drugs is NOT metabolized by the liver?

Accepted Answer

Penicillin G

Answer

Phenytoin

Answer

Erythromycin

Answer

Cimetidine

Question 10

A drug has 40% absorption and a hepatic extraction ratio of 0.6. What is the bioavailability of this drug?

Accepted Answer

16%

Answer

24%

Answer

20%

Answer

28%

Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics — MCQs

On this page

Practice by Chapter

Want unlimited practice?