Pharmacogenomics — MCQs

Pharmacogenomics Indian Medical PG Practice Questions and MCQs

Question 11: A drug that binds to a receptor at a site distinct from the active site and alters the affinity of the receptor for the endogenous ligand is a:

A. Competitive antagonist
B. Inverse agonist
C. Partial agonist
D. Allosteric modulator (Correct Answer)

Explanation: ### Explanation **1. Why Allosteric Modulator is Correct:** The term **"allosteric"** (Greek: *allos* = other, *stereos* = space) refers to a binding site on a receptor that is topographically distinct from the orthosteric (active) site where the endogenous ligand binds [1]. * **Mechanism:** When an allosteric modulator binds, it induces a conformational change in the receptor [1]. This change can either increase (**Positive Allosteric Modulator/PAM**) or decrease (**Negative Allosteric Modulator/NAM**) the affinity or efficacy of the endogenous ligand for its active site [1]. Since the question specifies binding at a distinct site to alter affinity, it perfectly describes an allosteric modulator. **2. Why Other Options are Incorrect:** * **Competitive Antagonist:** These bind to the **same active site** as the endogenous ligand. They compete for the same "space," and their effect can be overcome by increasing the concentration of the agonist. * **Inverse Agonist:** These bind to the same receptor (often the active site) but produce an effect **opposite** to that of the agonist. They reduce the constitutive (basal) activity of the receptor. * **Partial Agonist:** These bind to the active site but produce only a sub-maximal response, even at 100% receptor occupancy [2]. They have an intrinsic activity between 0 and 1. **3. High-Yield Clinical Pearls for NEET-PG:** * **Classic Example:** **Benzodiazepines** are PAMs of the $GABA_A$ receptor. They do not open the channel themselves but increase the frequency of channel opening in the presence of GABA. * **Cinacalcet:** A PAM at the calcium-sensing receptor used in hyperparathyroidism. * **Maraviroc:** An allosteric antagonist (NAM) of the CCR5 receptor used in HIV treatment. * **Key Distinction:** Unlike competitive antagonists, allosteric modulators do not compete for the active site [1], meaning they can saturate their effect regardless of agonist concentration (ceiling effect), often making them safer in overdose.

Question 12: What is true about pKa?

A. The pH at which the ionized fraction of a drug equals its unionized fraction. (Correct Answer)
B. The pH at which the ionized fraction of a drug is greater than its unionized fraction.
C. The pH at which the ionized fraction of a drug is less than its unionized fraction.
D. The pH at which the ionized fraction of a drug is twice its unionized fraction.

Explanation: **Explanation:** The concept of **pKa** is fundamental to understanding drug absorption and distribution. [1] By definition, pKa is the negative logarithm of the acid dissociation constant ($K_a$). [1] It represents the specific **pH at which a drug exists in a state of equilibrium**, where exactly **50% of the drug is ionized (charged) and 50% is unionized (uncharged).** [1] This relationship is governed by the **Henderson-Hasselbalch equation**: [1] $pH = pKa + eg \log \frac{[Ionized]}{[Unionized]}$ (for acids) When $pH = pKa$, the log term becomes $\log(1)$, which is zero, confirming that the concentrations of ionized and unionized forms are equal. [1] **Analysis of Options:** * **Option A (Correct):** Accurately describes the equilibrium state where ionized fraction = unionized fraction. [1] * **Options B, C, and D:** These represent states where the pH has shifted away from the pKa. According to the principle, for a **weakly acidic drug**, if the pH is higher than the pKa, the ionized fraction increases. [1] For a **weakly basic drug**, if the pH is lower than the pKa, the ionized fraction increases. [2] **High-Yield Clinical Pearls for NEET-PG:** 1. **Lipid Solubility:** Only the **unionized** form of a drug is lipid-soluble and can cross biological membranes (cell membranes, blood-brain barrier). [1] 2. **Ion Trapping:** This principle is used in treating toxicity. To excrete a **weakly acidic drug** (like Aspirin), we **alkalinize the urine** (using Sodium Bicarbonate). [1] This increases the ionized fraction in the renal tubules, "trapping" the drug in the urine and preventing reabsorption. [1] 3. **Acidic Drugs:** Bind primarily to **Albumin**. 4. **Basic Drugs:** Bind primarily to **$\alpha_1$-acid glycoprotein**.

Question 13: Which drugs should be avoided in G6PD deficiency?

A. Dapsone (Correct Answer)
B. Metformin
C. Pregabalin
D. Rituximab

Explanation: **Explanation:** **1. Why Dapsone is the Correct Answer:** Glucose-6-Phosphate Dehydrogenase (G6PD) is a critical enzyme in the pentose phosphate pathway that maintains the pool of **reduced glutathione** in red blood cells (RBCs). Reduced glutathione acts as an antioxidant, neutralizing free radicals and reactive oxygen species (ROS). **Dapsone** is a potent oxidizing agent. In G6PD-deficient individuals, the RBCs cannot regenerate enough reduced glutathione to combat the oxidative stress caused by Dapsone. This leads to the oxidation of hemoglobin into **Heinz bodies**, resulting in membrane damage and **acute hemolytic anemia**. **2. Analysis of Incorrect Options:** * **Metformin (B):** A biguanide used in Type 2 Diabetes. Its primary side effect is lactic acidosis; it does not cause oxidative stress or hemolysis. * **Pregabalin (C):** A gabapentinoid used for neuropathic pain and seizures. It has no known interaction with the G6PD enzyme pathway. * **Rituximab (D):** A monoclonal antibody against CD20. While it can cause infusion reactions or cytopenias, it is not an oxidizing drug and is safe in G6PD deficiency. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "AAA" Rule:** Common triggers for G6PD hemolysis include **A**ntimalarials (Primaquine, Chloroquine), **A**ntibiotics (Sulfonamides, Dapsone, Nitrofurantoin), and **A**ntipyretics (high-dose Aspirin). * **Fava Beans:** Ingestion causes "Favism," a classic trigger for hemolysis in these patients. * **Peripheral Smear Findings:** Look for **Heinz Bodies** (denatured hemoglobin) and **Bite Cells** (degmacytes) formed when splenic macrophages pluck out these bodies. * **Inheritance:** It is an **X-linked recessive** disorder, making it more common in males.

Question 14: Loading dose depends on the following factors except:

A. Drug concentration to be achieved
B. Volume of distribution
C. Clearance of the drug (Correct Answer)
D. Bioavailability of drug

Explanation: The **Loading Dose (LD)** is a large initial dose given to achieve the therapeutic plasma concentration ($C_p$) as quickly as possible. It is primarily determined by the **volume of distribution ($V_d$)**, not by how fast the drug is removed from the body [1, 2]. **1. Why Clearance (C) is the correct answer:** Clearance refers to the rate at which the drug is removed from the plasma. It determines the **Maintenance Dose (MD)**, which is required to maintain a steady-state concentration [1]. Since the loading dose is designed to "fill the tank" (the volume of distribution) rather than replace ongoing losses, clearance does not influence its calculation. **2. Analysis of Incorrect Options:** * **Volume of Distribution ($V_d$):** This is the most critical factor. The formula for LD is: $LD = \frac{V_d \times Target C_p}{F}$. A drug with a high $V_d$ (sequestered in tissues) requires a higher loading dose to saturate those tissues and reach the target plasma level [1, 2]. * **Target Drug Concentration ($C_p$):** The desired therapeutic level directly dictates how much drug must be administered [1]. * **Bioavailability ($F$):** For non-intravenous routes, the fraction of the drug that reaches systemic circulation must be accounted for. If $F$ is low, the administered dose must be higher [1]. --- ### High-Yield Clinical Pearls for NEET-PG * **The Golden Rule:** Loading dose depends on **Volume of Distribution ($V_d$)**; Maintenance dose depends on **Clearance ($CL$)**. * **Half-life ($t_{1/2}$):** It takes approximately **4 to 5 half-lives** to reach steady-state concentration without a loading dose. * **Clinical Application:** Loading doses are essential for drugs with long half-lives (e.g., **Amiodarone, Digoxin**) or in emergencies where immediate effect is needed (e.g., **Lidocaine** in arrhythmias). * **Renal/Hepatic Impairment:** In patients with renal failure, the **Maintenance Dose** must be reduced (due to decreased clearance), but the **Loading Dose** usually remains the same unless the $V_d$ is significantly altered.

Question 15: Variation in the sensitivity of response to increasing doses of a drug in different individuals can be obtained from which type of dose-response curve?

A. Graded dose-response curve
B. Quantal dose-response curve (Correct Answer)
C. Potency
D. Efficacy

Explanation: **Explanation:** The core of this question lies in distinguishing between how an individual responds to a drug versus how a population responds. **1. Why Option B is Correct:** A **Quantal Dose-Response Curve (QDRC)** describes an "all-or-none" response (e.g., sleep/no sleep, death/survival) across a population. It plots the fraction of the population that manifests a specific effect at increasing doses. Because individuals have different genetic makeups and physiological states (**inter-individual variation**), they require different doses to achieve the same effect. The QDRC is the primary tool used to visualize this variation in sensitivity and to determine the **Therapeutic Index (TI)**. **2. Why Other Options are Incorrect:** * **A. Graded Dose-Response Curve:** This measures the intensity of a response in a **single individual** as the dose increases (e.g., the increase in heart rate with increasing doses of Adrenaline). It defines efficacy and potency but does not account for population-wide sensitivity variations. * **C. Potency:** This refers to the amount of drug (dose) required to produce an effect of a given intensity. It is determined by the $EC_{50}$ on a Graded curve. * **D. Efficacy:** This refers to the maximal response ($E_{max}$) a drug can produce, regardless of dose. **NEET-PG High-Yield Pearls:** * **Graded Curve:** Tells us about **Efficacy** (Maximal effect) and **Potency** ($EC_{50}$). * **Quantal Curve:** Tells us about **Selectivity**, **Safety**, and **Variability** ($ED_{50}$, $TD_{50}$, $LD_{50}$). * **Therapeutic Index (TI):** Calculated from the Quantal curve as $TI = LD_{50} / ED_{50}$. A higher TI indicates a safer drug. * **Standard Safety Margin:** Also derived from the Quantal curve: $[(LD_1 - ED_{99}) / ED_{99}] \times 100$.

Question 16: What is the primary purpose of pharmacovigilance?

A. To regulate pharmacy companies
B. To monitor drug potency and efficacy
C. To monitor adverse effects of drugs (Correct Answer)
D. To ensure ethical drug practices

Explanation: **Explanation:** Pharmacovigilance (PV) is defined by the WHO as the science and activities relating to the **detection, assessment, understanding, and prevention of adverse effects** or any other drug-related problems. Its primary goal is to ensure patient safety by identifying previously unrecognized adverse drug reactions (ADRs) after a drug has been released into the market (Phase IV clinical trials). **Analysis of Options:** * **Option C (Correct):** The core mandate of pharmacovigilance is the continuous monitoring of the safety profile of drugs. Since clinical trials (Phases I-III) involve limited populations, rare or long-term adverse effects are often only discovered through post-marketing surveillance. * **Option A:** Regulating pharmaceutical companies is the role of national regulatory authorities (like the CDSCO in India or the FDA in the USA), not the specific process of pharmacovigilance. * **Option B:** While efficacy is monitored during clinical trials, pharmacovigilance focuses specifically on *safety* rather than how well the drug works (efficacy) or its chemical strength (potency). * **Option D:** Ethical drug practices are governed by Bioethics committees and Good Clinical Practice (GCP) guidelines. **High-Yield Clinical Pearls for NEET-PG:** * **Phase IV Clinical Trial:** This is synonymous with post-marketing surveillance and is the stage where pharmacovigilance is most active. * **Pharmacovigilance Programme of India (PvPI):** Launched in 2010; the National Coordinating Centre is the Indian Pharmacopoeia Commission (IPC), Ghaziabad. * **Uppsala Monitoring Centre (UMC):** Located in Sweden, it is the WHO collaborating centre for international drug monitoring. * **Spontaneous Reporting:** This is the most common method used in pharmacovigilance to identify new ADRs.

Question 17: Which of the following drugs is NOT bound to alpha-1-acid glycoprotein?

A. Lignocaine
B. Tolbutamide (Correct Answer)
C. Quinidine
D. Propranolol

Explanation: ### Explanation The binding of drugs to plasma proteins is a crucial pharmacokinetic concept. In the blood, drugs primarily bind to two types of proteins: **Albumin** and **Alpha-1-acid glycoprotein (AAG)**. **1. Why Tolbutamide is the Correct Answer:** Tolbutamide is an acidic drug (Sulfonylurea). As a general rule, **acidic drugs bind primarily to Plasma Albumin**, while **basic drugs bind to Alpha-1-acid glycoprotein (AAG)**. Since Tolbutamide is acidic, it does not bind to AAG, making it the correct choice. **2. Analysis of Incorrect Options:** * **Lignocaine (Option A):** This is a basic drug (local anesthetic) and is a classic example of a drug that binds extensively to AAG. * **Quinidine (Option B):** An alkaloid and a basic drug used as an antiarrhythmic; it binds significantly to AAG. * **Propranolol (Option D):** A basic beta-blocker that shows high affinity for AAG. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **The "Acid-to-Acid" Rule:** Remember **A**cidic drugs bind to **A**lbumin (e.g., NSAIDs, Warfarin, Phenytoin, Penicillins, and Sulfonylureas like Tolbutamide). * **The "Basic-to-Glycoprotein" Rule:** **B**asic drugs bind to **A**AG (e.g., Lidocaine, Propranolol, Quinidine, Tricyclic Antidepressants, and Bupivacaine). * **Clinical Significance of AAG:** AAG is an "acute-phase reactant." Its levels increase during inflammation, infection, or malignancy. This can lead to decreased free (active) fractions of basic drugs like Lignocaine, potentially requiring dosage adjustments in such patients. * **Albumin Levels:** Conversely, albumin levels often decrease in chronic liver disease or nephrotic syndrome, leading to increased toxicity of acidic drugs like Warfarin.

Question 18: Which drug has a wide therapeutic index?

A. Digoxin
B. Lithium
C. Phenytoin
D. Penicillin (Correct Answer)

Explanation: ### Explanation **Therapeutic Index (TI)** is a quantitative measurement of a drug's safety. It is defined as the ratio of the dose that produces toxicity to the dose that produces a clinically desired effect ($TI = TD_{50} / ED_{50}$). **1. Why Penicillin is Correct:** Penicillin has a **wide therapeutic index**, meaning there is a large margin between the effective dose and the toxic dose. This is primarily because penicillin targets the bacterial cell wall (peptidoglycan synthesis), a structure that does not exist in human cells. Consequently, even very high doses are generally well-tolerated by the human body, unless the patient has a specific hypersensitivity/allergy. **2. Why the Other Options are Incorrect:** * **Digoxin (A):** A classic example of a **narrow therapeutic index** drug. It requires therapeutic drug monitoring (TDM) because the dose required to treat heart failure/arrhythmias is very close to the dose that causes life-threatening digitalis toxicity. * **Lithium (B):** Used in bipolar disorder, lithium has an extremely narrow window (therapeutic range: 0.6–1.2 mEq/L). Levels above 1.5 mEq/L can lead to severe neurotoxicity and renal impairment. * **Phenytoin (C):** This antiepileptic follows **zero-order kinetics** (saturation kinetics) at higher therapeutic concentrations. Small dose increases can lead to disproportionately large increases in plasma levels, causing toxicity (ataxia, nystagmus). **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Narrow Therapeutic Index drugs:** "**W**arning, **T**hese **L**ethal **D**rugs **P**revent **C**uring" (**W**arfarin, **T**heophylline, **L**ithium, **D**igoxin, **P**henytoin/Phenobarbitone, **C**arbamazepine). * Drugs with a wide TI (e.g., Penicillin, Paracetamol, Benzodiazepines) are generally safer and do not require routine TDM. * **Low TI** = High risk of toxicity; **High TI** = Safer profile.

Question 19: What genetic variation in a drug metabolism pathway is known to cause severe toxicity with fluorouracil?

A. CYP2C9
B. Dihydropyrimidine dehydrogenase (Correct Answer)
C. Thiopurine-S methyltransferase
D. CYP2D6

Explanation: **Explanation:** **Dihydropyrimidine dehydrogenase (DPD)** is the rate-limiting enzyme responsible for the catabolism of over 80% of administered **5-Fluorouracil (5-FU)** and its oral prodrug, Capecitabine. In individuals with **DPD deficiency** (caused by mutations in the *DPYD* gene), the body cannot effectively clear the drug. This leads to an accumulation of active metabolites, resulting in severe, life-threatening toxicities such as profound myelosuppression, intractable diarrhea, neurotoxicity, and hand-foot syndrome. **Analysis of Incorrect Options:** * **CYP2C9:** This enzyme metabolizes drugs like **Warfarin**, Phenytoin, and NSAIDs. Variations lead to an increased risk of bleeding with warfarin. * **Thiopurine-S methyltransferase (TPMT):** This enzyme is responsible for the metabolism of 6-Mercaptopurine and Azathioprine. Deficiency leads to severe bone marrow toxicity when these drugs are used. * **CYP2D6:** This is a highly polymorphic enzyme involved in the metabolism of ~25% of drugs, including **Codeine** (conversion to morphine), Tamoxifen, and many antidepressants/antipsychotics. **High-Yield Clinical Pearls for NEET-PG:** * **DPD Testing:** Pre-therapeutic screening for *DPYD* variants is increasingly recommended to prevent fatal 5-FU toxicity. * **Uridine Triacetate:** This is the specific **antidote** used for 5-FU or Capecitabine overdose or early-onset severe toxicity. * **TPMT vs. DPD:** Do not confuse them; both cause chemo-toxicity, but TPMT is for thiopurines (6-MP) and DPD is for pyrimidines (5-FU).

Question 20: Which is the most appropriate definition of spare receptors?

A. Receptors that remain unoccupied even after a drug produces its maximal response. (Correct Answer)
B. Receptors involved in the binding of inverse agonists.
C. Receptors that are responsible for producing adverse drug effects.
D. Receptors that are unable to produce a maximum response.

Explanation: **Explanation:** The concept of **spare receptors** (also known as receptor reserve) is fundamental to pharmacodynamics [1]. A drug is said to have spare receptors when the **maximal biological response ($E_{max}$)** is achieved at a concentration where not all available receptors are occupied [1]. 1. **Why Option A is Correct:** In many systems, the relationship between receptor occupancy and tissue response is non-linear. Signal amplification (e.g., via G-proteins and second messengers like cAMP) allows a full agonist to trigger the maximum possible cellular effect while occupying only a small fraction (e.g., 1% to 10%) of the total receptor population [1]. The remaining unoccupied receptors are "spare." 2. **Why Other Options are Incorrect:** * **Option B:** Inverse agonists bind to the same receptor site as agonists but stabilize the inactive conformation, reducing constitutive activity. This is unrelated to the "spare" status. * **Option C:** Adverse effects are typically mediated by "off-target" binding or excessive activation of intended receptors, not specifically by spare receptors. * **Option D:** Receptors that cannot produce a maximum response even at 100% occupancy are associated with **partial agonists**, which have low intrinsic activity [1]. **High-Yield Clinical Pearls for NEET-PG:** * **EC50 vs. Kd:** In the presence of spare receptors, the **$EC_{50}$** (concentration for half-maximal response) is always **lower** than the **$K_d$** (concentration for half-maximal binding) [1]. * **Experimental Proof:** Spare receptors are demonstrated using **irreversible antagonists** [1]. If a small dose of an irreversible antagonist is added and the $E_{max}$ remains unchanged (though the curve shifts right), spare receptors exist. * **Clinical Significance:** Spare receptors increase the **sensitivity** of a cell to low concentrations of a drug, ensuring a robust physiological response even when ligand levels are low [1].

Practice by Chapter

Genetic Basis of Drug Response

Practice Questions

Pharmacogenomic Testing

Practice Questions

Cytochrome P450 Polymorphisms

Practice Questions

Pharmacogenomics of Drug Transporters

Practice Questions

Pharmacogenomics in Oncology

Practice Questions

Pharmacogenomics in Cardiovascular Therapeutics

Practice Questions

Pharmacogenomics in Psychiatry

Practice Questions

Personalized Medicine Approaches

Practice Questions

Ethical Considerations in Pharmacogenomics

Practice Questions

Implementation of Pharmacogenomics in Clinical Practice

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free