Pharmacokinetics and Pharmacodynamics — MCQs

Pharmacokinetics and Pharmacodynamics Indian Medical PG Practice Questions and MCQs

Question 141: Intracellular receptors are found in which of the following substances?

A. Insulin
B. Glucagon
C. Corticosteroids (Correct Answer)
D. All of the above

Explanation: **Explanation:** The location of a drug receptor is primarily determined by the **lipid solubility** of the ligand. **1. Why Corticosteroids are correct:** Corticosteroids (along with thyroid hormones, Vitamin D, and sex steroids) are highly **lipophilic**. This property allows them to easily cross the lipid bilayer of the cell membrane via simple diffusion. Once inside, they bind to **intracellular receptors** (specifically cytoplasmic receptors for steroids). The hormone-receptor complex then translocates to the nucleus, where it acts as a transcription factor, binding to the Hormone Response Element (HRE) on DNA to regulate gene expression. **2. Why other options are incorrect:** * **Insulin (Option A):** Insulin is a large peptide hormone. Being water-soluble and lipid-insoluble, it cannot cross the cell membrane. It binds to **Enzyme-linked receptors** (specifically Receptor Tyrosine Kinase) located on the cell surface. * **Glucagon (Option B):** Similar to insulin, glucagon is a peptide hormone. it acts via **G-Protein Coupled Receptors (GPCR)** on the cell surface, specifically using the adenylyl cyclase-cAMP second messenger system. **Clinical Pearls for NEET-PG:** * **Cytoplasmic Receptors:** Glucocorticoids, Mineralocorticoids, Progesterone, and Testosterone. * **Nuclear Receptors:** Thyroid hormones (T3/T4), Retinoic acid (Vitamin A), Vitamin D, and Estrogen. * **Speed of Action:** Receptors acting through gene transcription (Intracellular) have the **slowest onset of action** (hours to days) but the longest duration of effect compared to ion channels or GPCRs. * **Mnemonic:** "T-R-E-A-D" for Nuclear receptors: **T**hyroid, **R**etinoic acid, **E**strogen, **A**ldosterone (and other steroids), Vitamin **D**.

Question 142: Which of the following drugs is not administered sublingually?

A. Isosorbide dinitrate
B. Buprenorphine
C. Ergotamine tartrate
D. Isosorbide-5-mononitrate (Correct Answer)

Explanation: **Explanation:** The correct answer is **Isosorbide-5-mononitrate (ISMN)**. **Why Isosorbide-5-mononitrate is not given sublingually:** The primary rationale for sublingual administration is to bypass **first-pass metabolism** in the liver, ensuring rapid onset of action. Isosorbide-5-mononitrate is the active metabolite of Isosorbide dinitrate. It possesses **100% oral bioavailability** and is not subject to significant first-pass metabolism. Therefore, there is no clinical advantage to administering it sublingually; it is designed for oral use to provide a sustained effect for the prophylaxis of angina. **Analysis of Incorrect Options:** * **Isosorbide dinitrate (ISDN):** This drug undergoes extensive first-pass metabolism. The sublingual route is commonly used for the acute termination of an angina attack due to its rapid absorption into the systemic circulation. * **Buprenorphine:** This is a highly lipid-soluble opioid with high first-pass metabolism. Sublingual tablets are a standard formulation for treating opioid dependence and severe pain. * **Ergotamine tartrate:** Used in the treatment of acute migraine attacks, it is administered sublingually to ensure rapid absorption and to bypass the gastric stasis often associated with migraine. **High-Yield Clinical Pearls for NEET-PG:** * **Sublingual Route Advantages:** Rapid onset, bypasses first-pass metabolism, bypasses gastric acid, and can be self-terminated by spitting out the tablet. * **Nitrate Comparison:** * **GTN (Nitroglycerin):** Drug of choice for acute angina (Sublingual). * **ISDN:** Used for both acute (Sublingual) and chronic (Oral) management. * **ISMN:** Used only for chronic prophylaxis (Oral) due to its long half-life and excellent bioavailability. * **Other Sublingual Drugs:** Desmopressin, Nifedipine (historically for hypertensive emergencies, though now discouraged), and Fentanyl.

Question 143: Which of the following is a cause for less bioavailability?

A. High first pass metabolism (Correct Answer)
B. Increased absorption
C. Intravenous drug administration
D. High solubility

Explanation: **Explanation:** **Bioavailability (F)** is defined as the fraction of an administered dose of unchanged drug that reaches the systemic circulation. **1. Why Option A is Correct:** **First-pass metabolism** (or pre-systemic metabolism) occurs when a drug is metabolized in the gut wall or the liver before it reaches the systemic circulation. Drugs with high first-pass metabolism (e.g., Nitroglycerin, Propranolol, Morphine) undergo significant degradation immediately after absorption from the GI tract. This drastically reduces the amount of active drug entering the bloodstream, thereby lowering bioavailability. **2. Why the Other Options are Incorrect:** * **B. Increased absorption:** Higher absorption directly increases the amount of drug entering the portal or systemic circulation, which increases bioavailability. * **C. Intravenous (IV) drug administration:** By definition, IV administration bypasses the absorption phase and first-pass metabolism. It provides **100% bioavailability (F = 1)**, the highest possible value. * **D. High solubility:** For a drug to be absorbed, it must be lipid-soluble to cross biological membranes. High lipid solubility generally facilitates better absorption, leading to higher bioavailability. **High-Yield Clinical Pearls for NEET-PG:** * **Bioavailability Formula:** $F = \frac{\text{AUC (Oral)}}{\text{AUC (IV)}} \times 100$. * **Nitroglycerin:** Has such high first-pass metabolism that it is administered sublingually to bypass the liver and reach systemic circulation directly. * **Lidocaine:** Not given orally because its first-pass metabolism is so extensive that therapeutic levels cannot be safely achieved. * **Propranolol:** Shows significant inter-individual variation in bioavailability due to varying levels of hepatic enzymes.

Question 144: Most drugs are excreted by which route?

A. Feces
B. Urine (Correct Answer)
C. Saliva
D. None of the above

Explanation: **Explanation:** The correct answer is **Urine (Option B)**. The kidneys are the primary organs for drug excretion. Most drugs are filtered by the glomerulus or secreted into the renal tubules to be eliminated in the urine. For a drug to be excreted renally, it must be water-soluble (polar) [2]. Lipid-soluble drugs are typically metabolized by the liver into polar metabolites, which are then easily excreted by the kidneys [2]. This makes the renal route the most significant pathway for the systemic clearance of the majority of pharmacological agents. **Why other options are incorrect:** * **Feces (Option A):** While some drugs are excreted via bile into the feces (e.g., ceftriaxone, erythromycin) [1] or remain unabsorbed in the gut, this represents a smaller fraction compared to renal excretion. Biliary excretion is often followed by enterohepatic circulation, which actually prolongs the drug's stay in the body. * **Saliva (Option C):** Excretion via saliva, sweat, or tears is negligible in terms of total drug clearance. These routes are clinically significant only for specific diagnostic purposes (e.g., monitoring lithium or detecting drugs of abuse) or side effects (e.g., metallic taste). **High-Yield NEET-PG Pearls:** * **Glomerular Filtration Rate (GFR):** Only the "free" (unbound) fraction of a drug is filtered at the glomerulus. * **Ion Trapping:** To accelerate the excretion of acidic drugs (like Aspirin or Phenobarbitone), the urine is **alkalinized** with Sodium Bicarbonate [1]. To accelerate the excretion of basic drugs (like Amphetamines), the urine is **acidified** with Ammonium Chloride [1]. * **Zero-Order Kinetics:** Most drugs follow first-order kinetics, but a few (Phenytoin, Alcohol, Aspirin at high doses) follow zero-order kinetics, where a constant amount of drug is excreted per unit of time regardless of concentration.

Question 145: Which opioid has the maximum plasma protein binding capacity?

A. Morphine
B. Sufentanil (Correct Answer)
C. Fentanyl
D. Pethidine

Explanation: The degree of plasma protein binding (PPB) significantly influences the pharmacokinetics of opioids, particularly their volume of distribution and duration of action [1]. Among the options provided, **Sufentanil** has the highest plasma protein binding capacity. **1. Why Sufentanil is Correct:** Sufentanil is a highly potent synthetic opioid (5–10 times more potent than fentanyl) [2]. It exhibits a very high plasma protein binding of approximately **92.5%**, primarily to **alpha-1 acid glycoprotein**. This high protein binding, combined with its high lipid solubility, contributes to its rapid onset and specific redistribution profile. **2. Analysis of Incorrect Options:** * **Fentanyl:** While highly lipophilic, its protein binding is approximately **80–85%**, which is significantly lower than Sufentanil [2]. * **Pethidine (Meperidine):** This drug has moderate protein binding, typically around **60–70%** [2]. * **Morphine:** Morphine is the least protein-bound among the common opioids, with only about **30–35%** binding [2]. It is also the least lipophilic, leading to a slower onset of action across the blood-brain barrier compared to synthetic opioids. **3. NEET-PG High-Yield Pearls:** * **Potency Hierarchy:** Sufentanil > Fentanyl > Remifentanil > Morphine > Pethidine [2]. * **Protein Binding Target:** Most basic drugs (like opioids) bind to **Alpha-1 Acid Glycoprotein**, whereas acidic drugs (like NSAIDs or Warfarin) bind to **Albumin** [1]. * **Clinical Correlation:** High protein binding means that in states of hypoproteinemia (e.g., liver disease, malnutrition), the "free fraction" of the drug increases, potentially leading to toxicity even at standard doses. * **Remifentanil Note:** It is unique because it is metabolized by **non-specific plasma esterases**, giving it an ultra-short half-life regardless of infusion duration.

Question 146: Which class of drugs is highly bound to albumin?

A. Non-steroidal anti-inflammatory drugs (NSAIDs) (Correct Answer)
B. Lidocaine
C. Beta blockers
D. Verapamil

Explanation: **Explanation:** The binding of drugs to plasma proteins is a crucial pharmacokinetic parameter. The primary rule to remember for NEET-PG is that **acidic drugs bind to Albumin**, while **basic drugs bind to $\alpha_1$-acid glycoprotein (AAG)**. **1. Why NSAIDs are correct:** Non-steroidal anti-inflammatory drugs (NSAIDs), such as Ibuprofen, Naproxen, and Phenylbutazone, are **acidic drugs**. They have a high affinity for albumin (often >95% bound). Because they are highly protein-bound, they are susceptible to displacement interactions. For example, if two highly protein-bound acidic drugs are given together, one may displace the other, leading to a sudden increase in the free (active) fraction of the drug, potentially causing toxicity. **2. Why the other options are incorrect:** * **Lidocaine, Beta-blockers (e.g., Propranolol), and Verapamil** are all **basic drugs**. * Basic drugs primarily bind to **$\alpha_1$-acid glycoprotein (AAG)** and occasionally to lipoproteins. Therefore, they do not show significant binding to albumin compared to NSAIDs. **3. High-Yield Clinical Pearls for NEET-PG:** * **Albumin Binding (Acidic Drugs):** NSAIDs, Warfarin, Phenytoin, Penicillins, and Sulfonamides. * **$\alpha_1$-Acid Glycoprotein Binding (Basic Drugs):** Lidocaine, Quinidine, Bupivacaine, Propranolol, and Tricyclic Antidepressants (TCAs). * **Clinical Significance:** Only the **unbound (free) fraction** of a drug is pharmacologically active, metabolized, and excreted. * **Hypoalbuminemia:** In conditions like nephrotic syndrome or liver cirrhosis, the free fraction of acidic drugs (like Phenytoin) increases, necessitating dose adjustments to avoid toxicity.

Question 147: Atropine and acetylcholine show which type of antagonism?

A. Competitive (Correct Answer)
B. Noncompetitive
C. Both of the above
D. None of the above

Explanation: ### Explanation **1. Why Competitive Antagonism is Correct:** Competitive (reversible) antagonism occurs when the agonist and antagonist compete for the **same binding site** on the receptor [1]. Atropine is a classic competitive antagonist of **Muscarinic acetylcholine receptors** [3]. * **Mechanism:** Atropine binds to the muscarinic receptor, preventing Acetylcholine (ACh) from binding [3]. * **Surmountability:** Because they compete for the same site, the inhibitory effect of Atropine can be overcome (surmounted) by increasing the concentration of the agonist (ACh). This results in a **parallel rightward shift** of the dose-response curve without a change in the maximal response ($E_{max}$) [1]. **2. Why Other Options are Incorrect:** * **Noncompetitive Antagonism:** In this type, the antagonist binds to an **allosteric site** (different from the agonist site) or binds irreversibly to the active site. Increasing the agonist concentration cannot overcome this block, leading to a decrease in $E_{max}$ [2]. Atropine does not bind this way [2]. * **Both/None:** These are incorrect because the interaction between Atropine and ACh strictly follows the laws of competitive kinetics at muscarinic receptors. **3. NEET-PG High-Yield Pearls:** * **Key Feature:** Competitive antagonism increases the $K_m$ (decreases potency) but leaves $V_{max}$/$E_{max}$ unchanged [1]. * **Clinical Application:** In **Organophosphate poisoning** (where ACh levels are dangerously high due to acetylcholinesterase inhibition), Atropine is used as the specific pharmacological antagonist to compete with the excess ACh at muscarinic sites [3]. * **Other Examples:** Propranolol vs. Adrenaline (at \u03b2-receptors), Naloxone vs. Morphine (at \u03bc-receptors).

Question 148: Volume of distribution of a drug is given by which of the following equations?

A. Vd = dose administered i.v. / Total lipid solubility
B. Vd = maximum tolerated dose / Dose administered i.v.
C. Vd = dose administered i.v. / plasma concentration (Correct Answer)
D. Vd = t1/2 / Dose administered i.v.

Explanation: **Explanation:** **Volume of Distribution (Vd)** is a theoretical (apparent) volume that relates the total amount of drug in the body to the concentration of the drug in the plasma. It represents the extent to which a drug distributes into extravascular tissues versus the plasma. 1. **Why Option C is Correct:** The formula for Vd is derived from the principle of conservation of mass: **Vd = Amount of drug in the body / Plasma concentration (Cp)**. When a drug is administered intravenously (i.v.), the total amount in the body initially equals the dose administered. Therefore, **Vd = Dose (i.v.) / Cp**. A drug that stays in the blood has a low Vd, while a drug that sequesters in tissues (like fat or muscle) has a high Vd. 2. **Why Other Options are Incorrect:** * **Option A:** Lipid solubility affects *how* a drug distributes, but it is not a mathematical denominator for calculating volume. * **Option B:** This resembles the formula for the Therapeutic Index (LD50/ED50), not Vd. * **Option C:** Half-life ($t_{1/2}$) is related to Vd ($t_{1/2} = 0.693 \times Vd / CL$), but the ratio of $t_{1/2}$ to dose does not define Vd. **High-Yield Clinical Pearls for NEET-PG:** * **Loading Dose:** Vd is used to calculate the loading dose ($LD = Vd \times Target\ Cp$). * **Protein Binding:** Drugs with high plasma protein binding (e.g., Warfarin) have a **low Vd**. Drugs with high tissue binding (e.g., Digoxin, Chloroquine) have a **high Vd**. * **Dialysis:** Drugs with a very large Vd (e.g., TCAs) cannot be effectively removed by hemodialysis because most of the drug is outside the plasma. * **Chloroquine** has one of the highest Vd values (~13,000 L) due to extensive sequestration in tissues.

Question 149: Which of the following statements about drug absorption is false?

A. Glucose absorption through SGLT-1 is active.
B. Most drugs are absorbed by passive diffusion.
C. Acidic drugs are absorbed when pH < Pka. (Correct Answer)
D. Basic drugs are absorbed when pH > Pka.

Explanation: This question tests the fundamental principles of drug transport and the **pH Partition Hypothesis**. ### **Explanation of the Correct Answer (C)** The statement "Acidic drugs are absorbed when pH < pKa" is **false** because absorption depends on the **lipid solubility** of a drug. According to the Henderson-Hasselbalch principle: * **Acidic drugs** (like Aspirin) are **unionized** (lipid-soluble) in an **acidic medium** (pH < pKa). * **Basic drugs** (like Atropine) are **unionized** (lipid-soluble) in a **basic medium** (pH > pKa). While acidic drugs are indeed unionized in the stomach (pH < pKa), the statement is considered false in a clinical context because the **primary site of absorption for almost all oral drugs is the small intestine**, regardless of pH. This is due to the massive surface area provided by microvilli. Therefore, saying they are absorbed *only* or *primarily* because of the pH condition is a common misconception in pharmacokinetics. ### **Analysis of Other Options** * **Option A:** Correct. SGLT-1 (Sodium-Glucose Linked Transporter) uses **secondary active transport**, utilizing the sodium gradient to move glucose against its concentration gradient. * **Option B:** Correct. **Passive diffusion** is the most common mechanism for drug absorption, requiring no energy and moving drugs along a concentration gradient. * **Option D:** Correct. Basic drugs remain unionized and are better absorbed in alkaline environments (pH > pKa), such as the ileum. ### **NEET-PG High-Yield Pearls** * **Ion Trapping:** This principle is used to treat toxicity. To excrete an **acidic drug** (e.g., Phenobarbital or Salicylates), **alkalinize the urine** with Sodium Bicarbonate. This ionizes the drug, preventing reabsorption. * **Surface Area vs. pH:** The small intestine's surface area is ~200 $m^2$, making it the dominant absorption site even for acidic drugs that are technically more "ionized" there. * **P-glycoprotein (P-gp):** An efflux transporter that *reduces* drug absorption by pumping drugs back into the intestinal lumen.

Question 150: Which drug is not acetylated?

A. Isoniazid (INH)
B. Dapsone
C. Hydralazine
D. Metoclopramide (Correct Answer)

Explanation: ### Explanation The correct answer is **Metoclopramide**. **1. Underlying Medical Concept: Phase II Metabolism (Acetylation)** Acetylation is a major Phase II metabolic pathway catalyzed by the enzyme **N-acetyltransferase (NAT)** [2]. This process involves the transfer of an acetyl group to drugs containing an amino, hydroxyl, or sulfhydryl group. Genetic polymorphism in NAT leads to "Fast" and "Slow" acetylators, which significantly impacts drug toxicity and efficacy [1]. **2. Analysis of Options:** * **Metoclopramide (Correct):** This is a prokinetic and antiemetic drug. It is primarily metabolized in the liver via **glucuronidation and sulfation** (Phase II) and excreted in the urine. It does not undergo acetylation. * **Isoniazid (INH), Dapsone, and Hydralazine (Incorrect):** These drugs are classic examples of agents metabolized via acetylation [1]. Along with **Procainamide**, they are the most frequently tested drugs in this category. **3. High-Yield Clinical Pearls for NEET-PG:** To remember the drugs metabolized by acetylation, use the mnemonic **"SHIP"**: * **S** – Sulfonamides (including Dapsone) * **H** – Hydralazine * **I** – Isoniazid (INH) * **P** – Procainamide [1] **Key Exam Facts:** * **Drug-Induced Lupus Erythematosus (DILE):** Slow acetylators are at a significantly higher risk of developing DILE when taking Hydralazine, Procainamide, or INH [1]. * **Peripheral Neuropathy:** Slow acetylators taking INH are more prone to Vitamin B6 deficiency and subsequent neuropathy [1]. * **Dapsone:** Acetylation is the primary pathway; slow acetylators are at higher risk for hematological side effects like methemoglobinemia.

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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