Pharmacokinetics and Pharmacodynamics — MCQs

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?

Q224Medium

Which of the following is FALSE regarding competitive antagonism?

Q225Easy

Which of the following drugs primarily binds to albumin?

Q226Easy

What is the definition of pKa?

Q227Easy

Which of the following drugs is commonly administered by the intranasal route?

Q228Medium

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?

Q229Easy

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

Q230Medium

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?

Pharmacokinetics and Pharmacodynamics Indian Medical PG Practice Questions and MCQs

Question 221: Which of the following drugs exhibits zero-order kinetics?

A. Phenytoin (Correct Answer)
B. Phenobarbital
C. Phenobarbital
D. Digoxin

Explanation: **Explanation:** **Phenytoin** is a classic example of a drug that exhibits **Zero-Order Kinetics** (also known as non-linear or capacity-limited kinetics) at therapeutic or high concentrations. In zero-order kinetics, a constant *amount* of the drug is eliminated per unit of time, regardless of the plasma concentration. This occurs because the hepatic enzymes (CYP2C9/19) responsible for metabolizing phenytoin become saturated even at levels within the therapeutic range. Consequently, a small increase in dose can lead to a disproportionately large increase in plasma concentration, significantly increasing the risk of toxicity. **Analysis of Incorrect Options:** * **Phenobarbital & Digoxin:** These drugs follow **First-Order Kinetics**, which is the rule for the majority of drugs. In first-order kinetics, a constant *fraction* (percentage) of the drug is eliminated per unit of time. The rate of elimination is directly proportional to the plasma concentration. * *Note:* Option B and C are identical (Phenobarbital), both representing first-order elimination. **High-Yield Clinical Pearls for NEET-PG:** To remember the drugs following zero-order kinetics, use the mnemonic **"Zero WATTS P"**: * **W**arfarin (at very high doses) * **A**lcohol (Ethanol) - *Most common example* * **T**heophylline (at high doses) * **T**olbutamide * **S**alicylates (Aspirin - at high doses) * **P**henytoin **Key Distinction:** In first-order kinetics, the **half-life ($t_{1/2}$)** remains constant. In zero-order kinetics, the half-life is variable and increases as the dose/concentration increases.

Question 222: 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?

A. 0.5 L/hr
B. 2.0 L/hr (Correct Answer)
C. 5.0 L/hr
D. 20 L/hr

Explanation: <h3>Explanation</h3>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.

Question 223: 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?

A. 100 mg
B. 500 mg
C. 1 g
D. 2 g (Correct Answer)

Explanation: ### 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.

Question 224: Which of the following is FALSE regarding competitive antagonism?

A. The dose-response curve shifts to the right
B. The efficacy of the agonist decreases (Correct Answer)
C. The antagonism is reversible
D. The antagonist binds to the active site of the enzyme

Explanation: 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.

Question 225: Which of the following drugs primarily binds to albumin?

A. Steroids
B. Propranolol
C. Warfarin (Correct Answer)
D. Digoxin

Explanation: ### 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 226: What is the definition of pKa?

A. The pH at which a solute is completely ionized.
B. The pH at which the concentration of the ionized and unionized forms of a solute are equal. (Correct Answer)
C. The pH at which a solute is completely unionized.
D. All of the above.

Explanation: ### Explanation The concept of **pKa** is fundamental to understanding drug absorption and distribution. It is defined as the negative logarithm of the acid dissociation constant ($K_a$). **Why Option B is correct:** According to the **Henderson-Hasselbalch equation**: $$pH = pKa + \log \frac{[\text{Ionized}]}{[\text{Unionized}]}$$ When the pH of the medium is exactly equal to the pKa of the drug, the ratio of ionized to unionized forms becomes 1 (since $\log 1 = 0$). Therefore, at this specific pH, exactly **50% of the drug exists in the ionized form and 50% in the unionized form.** **Why other options are incorrect:** * **Options A & C:** Drugs are weak electrolytes; they exist in an equilibrium between ionized and unionized states. Complete ionization or unionization (100%) is a theoretical extreme that does not define the pKa. The degree of ionization shifts based on the pH of the environment relative to the drug's pKa. **NEET-PG High-Yield Clinical Pearls:** 1. **Lipid Solubility:** Only the **unionized** form of a drug is lipid-soluble and can cross biological membranes (e.g., GI tract, Blood-Brain Barrier). 2. **Ion Trapping:** This occurs when a drug moves to a compartment where the pH favors its ionized form, "trapping" it there. * *Clinical Application:* To treat **Aspirin (acidic drug) poisoning**, we alkalinize the urine with Sodium Bicarbonate. This increases the ionized fraction of aspirin in the renal tubules, preventing reabsorption and enhancing excretion. 3. **Rule of Thumb:** * Acidic drugs are better absorbed in acidic media (Stomach). * Basic drugs are better absorbed in basic media (Intestine).

Question 227: Which of the following drugs is commonly administered by the intranasal route?

A. Adrenaline
B. Desmopressin (Correct Answer)
C. Ganirelix
D. Insulin

Explanation: **Explanation:** **Desmopressin (Option B)** is the correct answer. It is a synthetic analogue of the antidiuretic hormone (Vasopressin). The intranasal route is a preferred non-invasive method for administering peptide drugs like Desmopressin because the nasal mucosa is highly vascularized, allowing for rapid absorption and bypassing first-pass metabolism. It is commonly used intranasally for the treatment of **Central Diabetes Insipidus** and was historically used for nocturnal enuresis (though oral forms are now preferred for the latter to reduce the risk of hyponatremia). **Analysis of Incorrect Options:** * **Adrenaline (Option A):** Typically administered Intramuscularly (IM) for anaphylaxis or Intravenously (IV) during cardiac arrest. While it can be used topically (e.g., to control epistaxis), it is not a standard "intranasal drug" for systemic effect. * **Ganirelix (Option C):** A GnRH antagonist used in assisted reproduction to prevent premature LH surges. It is administered via **Subcutaneous (SC)** injection. * **Insulin (Option D):** Primarily administered **Subcutaneously**. While an inhaled form (Afrezza) exists for pulmonary delivery, it is not administered via the intranasal route. **High-Yield Clinical Pearls for NEET-PG:** * **Other Intranasal Drugs:** Calcitonin (for osteoporosis), Midazolam (for acute seizures), Sumatriptan (for migraines), and Naloxone (for opioid overdose). * **Desmopressin Specifics:** It acts selectively on **V2 receptors**. Unlike natural Vasopressin, it lacks significant V1 activity, meaning it does not cause vasoconstriction (pressor effect). * **Drug of Choice:** Desmopressin is the DOC for Central Diabetes Insipidus and von Willebrand Disease (Type 1).

Question 228: 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?

A. Metabolism of drug in the liver
B. High plasma protein binding of the drug
C. Excretion of drug by the kidney
D. Redistribution of the drug (Correct Answer)

Explanation: ### Explanation The correct answer is **D. Redistribution of the drug.** **1. Why Redistribution is Correct:** The drug described is highly lipid-soluble and acts on the brain (a highly vascular organ). Upon intravenous injection, the drug rapidly reaches the brain, inducing anesthesia. However, its effect terminates quickly (15 minutes) because the drug moves out of the brain and "redistributes" into less vascular but more voluminous tissues like skeletal muscle and eventually adipose tissue. The termination of action for highly lipid-soluble drugs (like **Thiopentone** or **Propofol**) is due to **redistribution**, not metabolism or excretion. The drug remains in the body, but its concentration in the target organ (brain) falls below the therapeutic threshold. **2. Why Other Options are Incorrect:** * **A. Metabolism in the liver:** The question states the metabolism rate is only 10% per hour. If the drug lasted only 15 minutes due to metabolism, the rate would have to be much higher. Metabolism usually determines the *elimination half-life*, not the initial termination of action for these drugs. * **B. High plasma protein binding:** Protein binding generally acts as a reservoir and tends to *prolong* the duration of action by slowing down the drug's distribution and elimination. * **C. Excretion by the kidney:** Highly lipid-soluble drugs are not easily excreted by the kidney; they are filtered but then reabsorbed in the renal tubules. They must be metabolized into water-soluble metabolites before excretion. **3. NEET-PG High-Yield Pearls:** * **Redistribution** is the characteristic feature of highly lipid-soluble drugs (e.g., Thiopentone, Propofol, Fentanyl). * **Site of Redistribution:** Initially from Brain/Heart (Highly perfused) → Skeletal Muscle → Adipose Tissue (Poorly perfused). * **Clinical Correlation:** In obese patients, repeated doses of Thiopentone can lead to a "cumulative effect" because the adipose tissue reservoir becomes saturated, prolonging recovery time.

Question 229: High hepatic first-pass metabolism is seen in all EXCEPT?

A. Insulin (Correct Answer)
B. Propranolol
C. Lignocaine
D. Nitroglycerin

Explanation: The concept of **First-Pass Metabolism** refers to the extensive metabolism of a drug in the liver (or gut wall) before it reaches the systemic circulation, significantly reducing its oral bioavailability [2]. **Why Insulin is the Correct Answer:** Insulin is a **peptide hormone**. If administered orally, it is not primarily metabolized by the liver's first-pass effect; rather, it is **degraded by proteolytic enzymes** (pepsin, trypsin) and gastric acid in the gastrointestinal tract. Because it is destroyed in the gut before absorption even occurs, it cannot undergo hepatic first-pass metabolism in the traditional sense [1]. Under physiological conditions (endogenous), the liver normally clears approximately 60% of insulin, whereas exogenous subcutaneous insulin bypasses this initial portal passage [1]. This is why insulin must be given parenterally (subcutaneously or intravenously). **Analysis of Incorrect Options:** * **Propranolol:** A classic example of a drug with high hepatic extraction. It requires significantly higher oral doses compared to intravenous doses to achieve therapeutic levels. * **Lignocaine:** It has such an extensive first-pass metabolism (nearly 70%) that it is practically ineffective when given orally; hence, it is administered parenterally for arrhythmias. * **Nitroglycerin (GTN):** It undergoes near-total (90%) hepatic first-pass metabolism. This is why it is administered **sublingually**, bypassing the portal circulation to reach the systemic circulation directly. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for High First-Pass Metabolism:** "**L**ive **H**appily **P**rinting **M**oney **N**ow" (**L**ignocaine, **H**aloperidol/ **H**ormones (Testosterone), **P**ropranolol/ **P**ethidine, **M**orphine, **N**itroglycerin) [3]. * **Bioavailability (F):** Drugs with high first-pass metabolism have low oral bioavailability. $F = 1 - ER$ (where ER is the Extraction Ratio) [2]. * **Route Bypass:** Sublingual, transdermal, and rectal (partial) routes are used to bypass the first-pass effect [4].

Question 230: 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?

A. 200 mg
B. 100 mg
C. 0 mg
D. Cannot be predicted from this information (Correct Answer)

Explanation: ### Explanation **1. Why the correct answer is right (The Concept of Mixed-Order Kinetics)** Phenytoin exhibits **Michaelis-Menten kinetics** (also known as capacity-limited or non-linear kinetics) [1]. At low concentrations, it follows first-order kinetics (constant fraction eliminated). However, as the dose increases, the metabolic enzymes (CYP2C9) become saturated, and the drug shifts to **zero-order kinetics** (constant amount eliminated) [1], [2]. In this question, we are told the rate of elimination is 200 mg/hr at a body load of 400 mg. If the drug followed pure zero-order kinetics, 400 mg would be eliminated in 2 hours (leaving 0 mg). However, as the plasma concentration drops from 400 mg toward lower levels, the elimination kinetics will **shift from zero-order back to first-order** [2]. Because the exact concentration at which this "switch" occurs (the $K_m$ value) is not provided, and the rate of elimination will decrease as the concentration falls, the exact amount remaining cannot be calculated. **2. Why the other options are wrong** * **Option A (200 mg):** This assumes a constant elimination of 100 mg/hr, which contradicts the given rate of 200 mg/hr. * **Option B (100 mg):** This assumes a fixed half-life of 1 hour (first-order kinetics), which is incorrect for Phenytoin at saturable doses [2]. * **Option C (0 mg):** This assumes the drug stays in zero-order kinetics (200 mg/hr) until it is completely gone. In reality, the rate of elimination slows down as enzymes become unsaturated, meaning some drug will still remain. **3. High-Yield Clinical Pearls for NEET-PG** * **Drugs following Zero-Order Kinetics (Mnemonic: "Zero WATTS"):** **W**arfarin (at toxic doses), **A**lcohol/Aspirin, **T**heophylline, **T**olbutamide, **S**phenytoin (Phenytoin). * **Phenytoin Toxicity:** Small dosage increases can lead to disproportionately large increases in plasma levels due to enzyme saturation, leading to nystagmus, ataxia, and gum hypertrophy [2]. * **Therapeutic Window:** Phenytoin has a narrow therapeutic index (10–20 µg/mL) [2].

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