Pharmacokinetics and Pharmacodynamics Practice Questions

Q: Elimination after 3 half-lives in first-order kinetics is:

87.50%. ### Explanation **1. Understanding the Correct Answer (87.50%)** In **First-Order Kinetics**, a constant *fraction* of the drug is eliminated per unit of time. This means that with every half-life ($t_{1/2}$), the concentration of the drug remaining in the plasma is reduced by exactly 50%. To calculate the amount eliminated, we track the amount **remaining** first: * **After 1 $t_{1/2}$:** 50% remains (50% eliminated). * **After 2 $t_{1/2}$:** 50% of 50% = 25% remains (75% eliminated). * **After 3 $t_{1/2}$:** 50% of 25% = **12.5% remains**. To find the total amount eliminated: $100\% - 12.5\% = \mathbf{87.5\%}$. **2. Analysis of Incorrect Options** * **A. 12.50%:** This represents the amount of drug **remaining** in the body after 3 half-lives, not the amount eliminated. * **B. 75%:** This is the amount eliminated after **2 half-lives**. * **D. 94%:** This is the approximate amount eliminated after **4 half-lives** (specifically 93.75%). **3. NEET-PG High-Yield Pearls** * **Steady State:** It takes approximately **4 to 5 half-lives** to reach steady-state concentration ($C_{ss}$) during constant drug administration, and similarly 4 to 5 half-lives to completely eliminate a drug (>95%) after stopping it. * **First-Order vs. Zero-Order:** Most drugs follow first-order kinetics. In **Zero-Order kinetics** (e.g., Ethanol, high-dose Aspirin, Phenytoin), a constant *amount* (not fraction) is eliminated, and the concept of a fixed half-life does not apply. * **Formula for Half-life:** $t_{1/2} = 0.693 \times V_d / CL$. Note that half-life is independent of the dose in first-order kinetics.

Q: What percentage of a drug remains in the body after 3 half-lives?

12.50%. ### Explanation **Concept: First-Order Kinetics and Half-Life ($t_{1/2}$)** Most drugs follow first-order kinetics, where a constant **fraction** of the drug is eliminated per unit of time. The half-life is the time required for the plasma concentration of a drug to reduce by 50%. To calculate the percentage of drug remaining after $n$ half-lives, use the formula: **Remaining % = $(1/2)^n \times 100$** * **After 1 half-life:** $100\% \div 2 = 50\%$ remains. * **After 2 half-lives:** $50\% \div 2 = 25\%$ remains. * **After 3 half-lives:** $25\% \div 2 = \mathbf{12.5\%}$ **remains.** (Correct Answer) --- ### Analysis of Options * **Option A (12.50%):** Correct. As calculated above, after three cycles of 50% reduction, 12.5% of the original dose is still present in the systemic circulation. * **Option B (75%):** Incorrect. This represents the amount of drug **eliminated** after 2 half-lives ($100\% - 25\%$). * **Option C (87.50%):** Incorrect. This is the amount of drug **eliminated** after 3 half-lives ($100\% - 12.5\%$). * **Option D (94%):** Incorrect. This is approximately the amount of drug **eliminated** after 4 half-lives (93.75%). --- ### High-Yield NEET-PG Pearls 1. **Steady State:** It takes **4 to 5 half-lives** for a drug to reach steady-state concentration ($C_{ss}$) during constant infusion, and similarly, 4 to 5 half-lives for a drug to be considered "completely" eliminated from the body. 2. **Fixed Fraction vs. Fixed Amount:** In first-order kinetics, a **constant fraction** is eliminated. In zero-order kinetics (e.g., high-dose Aspirin, Alcohol, Phenytoin), a **constant amount** is eliminated regardless of plasma concentration. 3. **Rule of 7:** After 7 half-lives, more than 99% of the drug is eliminated.

Q: Rate of elimination of a new drug is 20 mg/hr at a steady state plasma concentration of 10 mg/L, then its renal clearance will be?

2.0 L/hr. Explanation 1. Understanding the Correct Answer (Option B: 2.0 L/hr) The core concept here is the relationship between the Rate of Elimination , Clearance (CL) , and Plasma Concentration (Cp) . Clearance is defined as the volume of plasma from which a drug is completely removed per unit of time. The mathematical formula is: $\text{Clearance (CL)} = \frac{\text{Rate of Elimination}}{\text{Plasma Concentration (Cp)}}$ [1] Plugging in the values from the question: * Rate of Elimination = 20 mg/hr * Plasma Concentration = 10 mg/L * $CL = \frac{20 \text{ mg/hr}}{10 \text{ mg/L}} = \mathbf{2.0 \text{ L/hr}}$ 2. Why Other Options are Incorrect * Option A (0.5 L/hr): This is the result of dividing concentration by the rate (10/20), which is the inverse of the correct formula. * Option C (5.0 L/hr): This value does not correlate with the provided data and suggests a calculation error. * Option D (20 L/hr): This assumes the clearance is equal to the rate of elimination, ignoring the plasma concentration factor. 3. Clinical Pearls & High-Yield Facts for NEET-PG * First-Order Kinetics: Most drugs follow first-order kinetics, where clearance remains constant regardless of the plasma concentration [1, 4]. * Zero-Order Kinetics: For drugs like Phenytoin, Alcohol, and Aspirin (at high doses), the rate of elimination is constant, but clearance decreases as plasma concentration increases (Saturation kinetics) [1, 4]. * Steady State: At steady state, the Rate of Drug Administration (Maintenance Dose) equals the Rate of Elimination. * Formula Shortcut: Remember that $Maintenance Dose = CL \times Cp_{ss}$. This is a frequent calculation target in NEET-PG.

Q: A 40-year-old man was brought to the ER after ingesting an unknown quantity of phenobarbital, the plasma level of which was 50 mg/L on admission. Pharmacokinetic parameters for phenobarbital are: Vd=40 L, CL=6 L/day, half-life = 4 days, oral bioavailability f=1. What was the approximate quantity of the drug that the patient ingested?

2 g. ### Explanation **1. Why the Correct Answer is Right (The Underlying Concept)** The question asks for the total quantity of the drug in the body (the loading dose or total body load) at the time of admission. This is calculated using the fundamental pharmacokinetic relationship between **Volume of Distribution ($V_d$)**, **Plasma Concentration ($C$)**, and **Amount of Drug ($Q$)**: $$\text{Amount of Drug (Q)} = V_d \times \text{Plasma Concentration (C)}$$ **Calculation:** * Given $V_d = 40\text{ L}$ * Given Plasma Concentration ($C$) = $50\text{ mg/L}$ * $Q = 40\text{ L} \times 50\text{ mg/L} = 2000\text{ mg}$ * $2000\text{ mg} = \mathbf{2\text{ g}}$ Since the oral bioavailability ($f$) is 1 (100%), the amount ingested is equal to the amount in the body. **2. Why Incorrect Options are Wrong** * **Option A (100 mg) & B (500 mg):** These values are significantly lower than the calculated body load. These doses would result in sub-therapeutic plasma levels ($2.5\text{ mg/L}$ and $12.5\text{ mg/L}$ respectively) for a drug with a $40\text{ L}$ $V_d$. * **Option C (1 g):** This would be the answer if the $V_d$ were $20\text{ L}$ or if the plasma concentration were $25\text{ mg/L}$. It represents a mathematical error in applying the formula. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Volume of Distribution ($V_d$):** It is a theoretical volume, not a physical one. Drugs with high $V_d$ (e.g., Digoxin, Chloroquine) are sequestered in tissues and cannot be removed effectively by hemodialysis. * **Phenobarbital Management:** It is a weak acid. In toxicity, **urinary alkalinization** (using IV Sodium Bicarbonate) is used to increase its excretion. This process is called "ion trapping." * **Zero-order vs. First-order:** While most drugs follow first-order kinetics (like phenobarbital here), remember **"WATT"** for Zero-order: **W**arfarin (at high doses), **A**lcohol/Aspirin, **T**heophylline, **T**olbutamide, and Phenytoin.

Q: Which of the following is FALSE regarding competitive antagonism?

The efficacy of the agonist decreases. In competitive antagonism, the antagonist competes with the agonist for the same binding site (active site) on the receptor. ### **Why Option B is FALSE (The Correct Answer)** In competitive antagonism, the **efficacy ($E_{max}$)** of the agonist remains **unchanged**. Because the binding is reversible, increasing the concentration of the agonist can "surmount" or displace the antagonist from the receptors. Once the agonist concentration is high enough, it can still achieve the maximum possible response. Therefore, the maximal effect ($E_{max}$) does not decrease. ### **Analysis of Other Options** * **Option A (True):** Because the antagonist competes for the site, a higher concentration of the agonist is required to produce the same effect. This increases the $EC_{50}$ (decreases potency), causing a **parallel rightward shift** of the log dose-response curve. * **Option C (True):** Competitive antagonism is **reversible**. The bond is usually non-covalent (ionic or Van der Waals), allowing the antagonist to dissociate when agonist concentration increases. * **Option D (True):** By definition, competitive antagonists bind to the **orthosteric site** (the same active site where the endogenous ligand or agonist binds). ### **NEET-PG High-Yield Pearls** * **Competitive Antagonism:** $V_{max}$ (or $E_{max}$) is constant; $K_m$ (or $EC_{50}$) increases. (Example: Atropine vs. Acetylcholine). * **Non-Competitive Antagonism:** $V_{max}$ (or $E_{max}$) decreases; $K_m$ (or $EC_{50}$) remains constant. This is usually irreversible or allosteric. (Example: Phenoxybenzamine at alpha receptors). * **Key Visual:** Look for a "parallel shift to the right" on a graph to identify competitive inhibition.

Q: Which of the following drugs primarily binds to albumin?

Warfarin. ### Explanation **Concept of Plasma Protein Binding** In pharmacokinetics, drugs circulate in the blood either bound to plasma proteins or in a free (active) form. The two most important proteins are **Albumin** (which primarily binds **acidic drugs**) and **$\alpha_1$-acid glycoprotein** (which primarily binds **basic drugs**). **Why Warfarin is Correct:** **Warfarin** is a highly acidic drug with a very high affinity for **Albumin** (approx. 99% bound). Because it is so extensively bound, any drug that displaces warfarin from albumin (like sulfonamides or NSAIDs) can significantly increase its free concentration, leading to a high risk of bleeding. This makes it a classic example of clinically significant protein-binding displacement interactions. **Analysis of Incorrect Options:** * **Steroids (Option A):** Endogenous steroids and many synthetic analogs primarily bind to specific globulins, such as **Corticosteroid Binding Globulin (CBG)** or Transcortin, rather than albumin. * **Propranolol (Option B):** This is a **basic drug**. Basic drugs typically bind to **$\alpha_1$-acid glycoprotein (AAG)**. Other examples include lidocaine and quinidine. * **Digoxin (Option C):** Digoxin has minimal plasma protein binding (approx. 25%). It primarily binds to tissue proteins (specifically Na+/K+ ATPase in cardiac and skeletal muscle), which accounts for its large volume of distribution ($V_d$). **High-Yield NEET-PG Pearls:** * **Acidic Drugs (Bind to Albumin):** Warfarin, NSAIDs, Phenytoin, Penicillins, Sulfonamides. * **Basic Drugs (Bind to $\alpha_1$-acid glycoprotein):** Propranolol, Lidocaine, Bupivacaine, Tricyclic Antidepressants (TCAs). * **Clinical Significance:** Only the **free fraction** of a drug is pharmacologically active, metabolized, and excreted. * **Hypoalbuminemia:** In conditions like nephrotic syndrome or cirrhosis, the dose of highly albumin-bound drugs (like Phenytoin) must be reduced to avoid toxicity.

Question 1

Drug X is normally administered at a rate of 50mg/hr. The elimination of Drug X from the body occurs via: Hepatic metabolism 10%, Biliary secretion 10%, Renal excretion 80%. This drug is to be administered to a 65-year-old patient with a GFR of 60 ml/min (assuming normal GFR is 120 ml/min). Liver and biliary functions are normal in the patient. What will be the dose rate of Drug X in this patient?

Accepted Answer

75mg/hr

Answer

50mg/hr

Answer

25mg/hr

Answer

100mg/hr

Question 2

Elimination after 3 half-lives in first-order kinetics is:

Accepted Answer

87.50%

Answer

12.50%

Answer

75%

Answer

94%

Question 3

What percentage of a drug remains in the body after 3 half-lives?

Accepted Answer

12.50%

Answer

75%

Answer

87.50%

Answer

94%

Question 4

Rate of elimination of a new drug is 20 mg/hr at a steady state plasma concentration of 10 mg/L, then its renal clearance will be?

Accepted Answer

2.0 L/hr

Answer

0.5 L/hr

Answer

5.0 L/hr

Answer

20 L/hr

Question 5

A 40-year-old man was brought to the ER after ingesting an unknown quantity of phenobarbital, the plasma level of which was 50 mg/L on admission. Pharmacokinetic parameters for phenobarbital are: Vd=40 L, CL=6 L/day, half-life = 4 days, oral bioavailability f=1. What was the approximate quantity of the drug that the patient ingested?

Accepted Answer

2 g

Answer

100 mg

Answer

500 mg

Answer

1 g

Question 6

Which of the following is true about tachyphylaxis?

Accepted Answer

It involves indirect sympathomimetics.

Answer

It involves direct sympathomimetics.

Answer

Its mechanism is clearly understood.

Answer

Tachyphylaxis of ephedrine is reversed with dopamine.

Question 7

Which of the following is FALSE regarding competitive antagonism?

Accepted Answer

The efficacy of the agonist decreases

Answer

The dose-response curve shifts to the right

Answer

The antagonism is reversible

Answer

The antagonist binds to the active site of the enzyme

Question 8

Which of the following drugs primarily binds to albumin?

Accepted Answer

Warfarin

Answer

Steroids

Answer

Propranolol

Answer

Digoxin

Question 9

A new drug is found to be highly lipid soluble. It has a metabolism rate of 10% per hour. On intravenous injection, it produces general anaesthesia that lasts only for 15 minutes. What is the likely reason for the short duration of anaesthesia?

Accepted Answer

Redistribution of the drug

Answer

Metabolism of drug in the liver

Answer

High plasma protein binding of the drug

Answer

Excretion of drug by the kidney

Question 10

High hepatic first-pass metabolism is seen in all EXCEPT?

Accepted Answer

Insulin

Answer

Propranolol

Answer

Lignocaine

Answer

Nitroglycerin

Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics — MCQs

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