Pharmacogenomics and Precision Medicine — MCQs
The cytochrome involved in monooxygenase-mediated detoxification of drugs is:
What is the primary role of Cytochrome P450 enzymes in the liver?
What is the mechanism of metabolism for alcohol, aspirin, and phenytoin at high doses?
A patient with history of ischemic stroke was started on clopidogrel. However, she had another attack of stroke after 6 months. Which of the following is likely to be responsible for the failure of clopidogrel in this patient?
Which of the following cytochromes is involved in monooxygenase mediated detoxification of drugs?
A diabetic female on Isoniazid (INH) and Rifampicin for tuberculosis developed Deep Vein Thrombosis (DVT). She was started on Warfarin, but her Prothrombin Time (PT) is not elevated. What is the next appropriate step in management?
Which antibiotic is MOST frequently implicated as a cause of drug-induced liver injury (DILI)?
Among the drugs used for Hepatitis B antiviral therapy, which of the following has the highest tendency for peripheral neuropathy?
Which of the following drugs should not be given in renal failure?
A 60-year-old male presents with muscle pain, fatigue, and dark urine. His past medical history is significant for stable angina, and his current medications include atenolol, atorvastatin, and aspirin. His urine dipstick test is positive for blood, but urine microscopy did not reveal RBCs. Serum creatinine kinase was significantly elevated. Which of the following medications, if added to his regimen, is most likely to have precipitated this patient's condition?
Pharmacogenomics and Precision Medicine Indian Medical PG Practice Questions and MCQs
Question 1: The cytochrome involved in monooxygenase-mediated detoxification of drugs is:
- A. Cyt P 450 (Correct Answer)
- B. Cytochrome b5
- C. Cytochrome c
- D. Cytochrome oxidase
Explanation: ***Cyt P 450*** - **Cytochrome P450 (CYP450)** enzymes are a superfamily of heme-containing monooxygenases primarily responsible for the **metabolism of xenobiotics**, including the detoxification of drugs. - They catalyze oxidation reactions, introducing a hydroxyl group to substrates, which typically increases their **hydrophilicity** and facilitates excretion. *Cytochrome c* - **Cytochrome c** is a component of the **electron transport chain** in mitochondria, primarily involved in cellular respiration and energy production. - It acts as an **electron carrier** between Complex III and Complex IV, not directly in drug detoxification. *Cytochrome b5* - **Cytochrome b5** participates in various metabolic reactions, including **fatty acid desaturation** and cholesterol biosynthesis, and can sometimes assist CYP450 enzymes. - However, it does not function as a primary monooxygenase for drug detoxification itself. *Cytochrome oxidase* - **Cytochrome oxidase** (Complex IV) is the terminal enzyme in the **electron transport chain**, responsible for the reduction of oxygen to water. - Its main role is in cellular respiration, and it is not directly involved in drug monooxygenation or detoxification.
Question 2: What is the primary role of Cytochrome P450 enzymes in the liver?
- A. Lipid transport
- B. Oxidation of drugs (Correct Answer)
- C. Carbohydrate synthesis
- D. Protein degradation
Explanation: ***Oxidation of drugs*** - **Cytochrome P450 enzymes** are a superfamily of monooxygenases that primarily catalyze the **oxidation of various endogenous and exogenous substrates**, including drugs [1, 2]. - This oxidative metabolism is a key step in detoxification and elimination of foreign compounds from the body [1]. *Lipid transport* - **Lipid transport** is primarily facilitated by **lipoproteins** and specific **transport proteins** in the blood and within cells. - While P450 enzymes can metabolize some lipids, their primary role is not in lipid transport [2]. *Carbohydrate synthesis* - **Carbohydrate synthesis**, or **gluconeogenesis**, is mainly carried out by enzymes such as **pyruvate carboxylase** and **fructose-1,6-bisphosphatase**. - Cytochrome P450 enzymes do not play a direct role in the synthesis of carbohydrates. *Protein degradation* - **Protein degradation** is largely mediated by the **ubiquitin-proteasome system** and **lysosomal pathways**. - Cytochrome P450 enzymes are not directly involved in breaking down proteins into smaller peptides or amino acids.
Question 3: What is the mechanism of metabolism for alcohol, aspirin, and phenytoin at high doses?
- A. First pass kinetics
- B. First order kinetics
- C. Zero order kinetics (Correct Answer)
- D. Second order kinetics
Explanation: ***Zero order kinetics*** - This mechanism occurs when the **metabolic enzymes become saturated at high drug concentrations**, leading to a constant amount (not a constant percentage) of drug being eliminated per unit time. - Alcohol, aspirin, and phenytoin are examples of drugs that exhibit **saturable metabolism**, transitioning from first-order to zero-order kinetics at higher doses. *First pass kinetics* - This describes the **metabolism of a drug by the liver or gut wall enzymes before it reaches systemic circulation** after oral administration. - While relevant to the oral bioavailability of these drugs, it does not describe the specific mechanism of elimination at high doses. *First order kinetics* - In this mechanism, a **constant fraction or percentage of the drug is eliminated per unit of time**, meaning the rate of elimination is directly proportional to the drug concentration. - Most drugs follow first-order kinetics at therapeutic doses because metabolizing enzymes are not saturated. *Second order kinetics* - This is a **less common pharmacokinetic model** where the rate of elimination is proportional to the square of the drug concentration or involves two reactants. - It does not typically describe the common elimination patterns of most drugs, including alcohol, aspirin, and phenytoin.
Question 4: A patient with history of ischemic stroke was started on clopidogrel. However, she had another attack of stroke after 6 months. Which of the following is likely to be responsible for the failure of clopidogrel in this patient?
- A. Upregulation of CYP1A1
- B. Downregulation of CYP2E1
- C. Downregulation of CYP2C19 (Correct Answer)
- D. Downregulation of CYP2D6
Explanation: ***Reduced function/Loss of function of CYP2C19*** - **Clopidogrel** is a **prodrug** that requires activation by **hepatic cytochrome P450 (CYP) enzymes**, primarily **CYP2C19**, to its active metabolite. - **Genetic polymorphisms** causing **reduced function or loss of function of CYP2C19** (e.g., CYP2C19*2, *3 alleles) result in insufficient conversion of clopidogrel to its active form, leading to **clopidogrel resistance** and increased risk of thrombotic events like recurrent stroke. - These **poor metabolizers** have significantly reduced antiplatelet response to standard clopidogrel doses. *Upregulation of CYP1A1* - **CYP1A1** is involved in the metabolism of various xenobiotics but plays a **minimal role** in clopidogrel activation. - Upregulation of CYP1A1 would not be a primary factor in clopidogrel failure as it is not the main enzyme responsible for its bioactivation. *Downregulation of CYP2E1* - **CYP2E1** is primarily involved in the metabolism of small organic molecules, some drugs, and toxins, and has **no significant role** in the bioactivation of clopidogrel. - Therefore, changes in its expression would not impact clopidogrel's efficacy. *Downregulation of CYP2D6* - **CYP2D6** is a major enzyme involved in the metabolism of many psychoactive drugs, beta-blockers, and opioids, but plays only a **minor role** in clopidogrel activation compared to CYP2C19. - Downregulation of CYP2D6 would not be the primary cause of clopidogrel failure.
Question 5: Which of the following cytochromes is involved in monooxygenase mediated detoxification of drugs?
- A. Cytochrome b5
- B. Cytochrome P450 (Correct Answer)
- C. Cytochrome c
- D. NADPH-cytochrome P450 reductase
Explanation: ***Cyt P 450*** - **Cytochrome P450** enzymes are a superfamily of **monooxygenases** that play a critical role in the metabolism and detoxification of a wide variety of endogenous and exogenous substances, including drugs. - They facilitate phase I reactions (e.g., **oxidation**, reduction, hydrolysis), which typically introduce or expose functional groups to make compounds more polar and easier to excrete. *Cytochrome b5* - **Cytochrome b5** is involved in various metabolic reactions, including **fatty acid desaturation** and cholesterol biosynthesis, and can sometimes interact with P450 systems but is not the primary monooxygenase for drug detoxification. - It also participates in the reduction of methemoglobin and can act as an electron donor, but its role in drug detoxification is secondary and accessory to P450. *Cytochrome c* - **Cytochrome c** is a key component of the **electron transport chain** in mitochondria, primarily involved in cellular respiration and ATP production. - It has a crucial role in **apoptosis** when released into the cytosol, but it is not directly involved in drug monooxygenase detoxification. *NADPH-cytochrome P450 reductase* - **NADPH-cytochrome P450 reductase** is an enzyme that transfers electrons from NADPH to **cytochrome P450 enzymes**, enabling their monooxygenase activity. - While essential for P450 function, it is the **reductase** (electron donor) and not the monooxygenase enzyme itself, which is Cytochrome P450.
Question 6: A diabetic female on Isoniazid (INH) and Rifampicin for tuberculosis developed Deep Vein Thrombosis (DVT). She was started on Warfarin, but her Prothrombin Time (PT) is not elevated. What is the next appropriate step in management?
- A. Increase the dose of Warfarin
- B. Replace Warfarin with Acenocoumarol
- C. Switch Ethambutol for Rifampicin
- D. Use Low Molecular Weight Heparin (LMWH) (Correct Answer)
Explanation: ### Explanation **1. Why LMWH is the Correct Choice:** The patient is taking **Rifampicin**, a potent **cytochrome P450 (CYP3A4 and CYP2C9) enzyme inducer**. Warfarin is metabolized by these enzymes; therefore, Rifampicin significantly increases Warfarin metabolism, leading to subtherapeutic levels and a failure to elevate the Prothrombin Time (PT/INR). In the acute management of DVT, achieving rapid and reliable anticoagulation is critical [1]. Since the drug interaction makes Warfarin unpredictable and difficult to titrate, switching to **Low Molecular Weight Heparin (LMWH)** is the most appropriate step. LMWH does not rely on the CYP450 system and provides immediate, predictable anticoagulation [2]. **2. Analysis of Incorrect Options:** * **Option A (Increase Warfarin dose):** While theoretically possible, the induction effect of Rifampicin is so profound that extremely high doses of Warfarin would be required, making the INR highly unstable and increasing the risk of toxicity if Rifampicin is later discontinued. * **Option B (Replace with Acenocoumarol):** Acenocoumarol is also a Vitamin K antagonist metabolized by the liver; it shares the same metabolic pathways and drug interactions as Warfarin [1]. * **Option C (Switch Ethambutol for Rifampicin):** Rifampicin is a cornerstone of Short Course Chemotherapy (SCC) for TB. It should not be discontinued or replaced with a less potent drug like Ethambutol solely to accommodate Warfarin, as this risks treatment failure or MDR-TB. **3. NEET-PG High-Yield Pearls:** * **Rifampicin:** The "Great Inducer." It reduces the efficacy of Warfarin, Oral Contraceptive Pills (OCPs), Sulfonylureas, and Digoxin. * **Isoniazid (INH):** Conversely, INH is a CYP enzyme **inhibitor**, but in this clinical scenario, the inducing effect of Rifampicin dominates. * **Management Rule:** When a patient on Rifampicin requires anticoagulation, LMWH or Fondaparinux are preferred over Vitamin K antagonists due to predictable pharmacokinetics [2].
Question 7: Which antibiotic is MOST frequently implicated as a cause of drug-induced liver injury (DILI)?
- A. Amoxicillin-clavulanic acid (Correct Answer)
- B. Oxacillin
- C. Erythromycin
- D. Doxycycline
Explanation: **Amoxicillin-clavulanic acid (Co-amoxiclav)** is the most common cause of drug-induced liver injury (DILI) worldwide. The injury is typically **idiosyncratic** and characterized by a **cholestatic** pattern (elevated alkaline phosphatase and bilirubin), though mixed patterns can occur [1]. Interestingly, the injury is specifically attributed to the **clavulanic acid** component rather than the amoxicillin itself. It usually manifests within 1–3 weeks after starting the drug, but can occur even after the course is completed. **Analysis of Incorrect Options:** * **Oxacillin:** While anti-staphylococcal penicillins (like Oxacillin and Nafcillin) can cause transient elevations in transaminases or hepatitis, they are statistically less frequent causes of DILI compared to Co-amoxiclav [2]. * **Erythromycin:** Classically associated with **cholestatic hepatitis** (especially the estolate formulation), it was historically a major cause of DILI. However, its usage has declined in favor of newer macrolides, and it remains less frequent than Co-amoxiclav in modern epidemiological studies. * **Doxycycline:** Tetracyclines are generally associated with microvesicular steatosis (fatty liver), particularly with high-dose IV administration in pregnant women. Doxycycline is rarely implicated in severe DILI. **High-Yield Clinical Pearls for NEET-PG:** * **Most common drug class for DILI:** Antibiotics (overall). * **Most common single agent for DILI:** Amoxicillin-clavulanic acid. * **Pattern of Injury:** Co-amoxiclav typically causes a **cholestatic** pattern (Pruritus + Jaundice) [1]. * **Risk Factors:** Older age and male sex increase the risk for Co-amoxiclav-induced DILI. * **Other common culprits:** Isoniazid (Hepatocellular/Necrotic pattern), Valproate (Microvesicular steatosis), and Nitrofurantoin (Autoimmune-like hepatitis).
Question 8: Among the drugs used for Hepatitis B antiviral therapy, which of the following has the highest tendency for peripheral neuropathy?
- A. Lamivudine
- B. Zidovudine
- C. Stavudine (Correct Answer)
- D. Nevirapine
Explanation: **Explanation:** The correct answer is **Stavudine (d4T)**. **1. Why Stavudine is correct:** Stavudine is a Nucleoside Reverse Transcriptase Inhibitor (NRTI). Its primary mechanism of toxicity involves the inhibition of **Mitochondrial DNA polymerase-gamma**. This leads to mitochondrial dysfunction, which clinically manifests as **distal symmetrical peripheral neuropathy** and lactic acidosis. Among the NRTIs, the "d-drugs" (Stavudine, Didanosine, and Zalcitabine) have the highest affinity for polymerase-gamma, making them the most notorious for causing nerve damage. **2. Analysis of Incorrect Options:** * **Lamivudine (3TC):** While also an NRTI used in both HIV and Hepatitis B, it is one of the least toxic drugs in its class. It has a very low affinity for mitochondrial DNA polymerase and rarely causes neuropathy. * **Zidovudine (AZT):** The hallmark side effect of Zidovudine is **bone marrow suppression** (anemia and neutropenia) and myopathy, rather than peripheral neuropathy. * **Nevirapine:** This is a Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI). Its most significant adverse effects are **hepatotoxicity** and severe skin rashes (including Stevens-Johnson Syndrome), not neuropathy. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Neuropathy:** Remember the **"3 Ds"** for peripheral neuropathy: **D**idanosine, **D**eoxycytidine (Zalcitabine), and **D**4T (Stavudine). * **Stavudine** is also strongly associated with **Lipoatrophy** (loss of subcutaneous fat from the face and limbs). * For Chronic Hepatitis B, **Tenofovir** and **Entecavir** are currently the preferred first-line agents due to high potency and a high genetic barrier to resistance.
Question 9: Which of the following drugs should not be given in renal failure?
- A. Clindamycin
- B. Methicillin (Correct Answer)
- C. Amoxicillin
- D. Rifampicin
Explanation: **Explanation:** The correct answer is **Methicillin**. The primary reason Methicillin is contraindicated in renal failure is its significant association with **Acute Interstitial Nephritis (AIN)** [1]. Methicillin is a potent nephrotoxin that can cause hypersensitivity-mediated renal damage, potentially worsening pre-existing renal impairment or causing acute kidney injury [3]. Due to this high risk of nephrotoxicity, Methicillin has largely been replaced in clinical practice by safer alternatives like Cloxacillin or Nafcillin. **Analysis of Options:** * **Methicillin (Correct):** It is primarily excreted by the kidneys and is the classic prototype drug for drug-induced AIN [1]. Its use is avoided in patients with renal compromise to prevent further parenchymal damage [3]. * **Clindamycin:** This drug is primarily metabolized by the **liver** and excreted in the bile. Therefore, it does not require dose adjustment in renal failure and is considered safe. * **Amoxicillin:** While Amoxicillin is renally excreted, it is not inherently nephrotoxic. In cases of renal failure, the **dose is adjusted** (decreased frequency), but the drug is not strictly contraindicated. * **Rifampicin:** This is primarily metabolized by the **liver** and eliminated via the fecal/biliary route. It is safe to use in renal failure without significant dose modification. **NEET-PG High-Yield Pearls:** 1. **Safe in Renal Failure (No dose adjustment):** Clindamycin, Erythromycin, Rifampicin, Doxycycline, and Ceftriaxone. 2. **Drugs causing AIN:** Penicillins (Methicillin), NSAIDs, Sulphonamides, and Diuretics [1]. 3. **Doxycycline** is the tetracycline of choice in renal failure because it is excreted via the gut (biliary). 4. **Aminoglycosides and Amphotericin B** are the most common causes of Acute Tubular Necrosis (ATN) in clinical settings [2].
Question 10: A 60-year-old male presents with muscle pain, fatigue, and dark urine. His past medical history is significant for stable angina, and his current medications include atenolol, atorvastatin, and aspirin. His urine dipstick test is positive for blood, but urine microscopy did not reveal RBCs. Serum creatinine kinase was significantly elevated. Which of the following medications, if added to his regimen, is most likely to have precipitated this patient's condition?
- A. Erythromycin (Correct Answer)
- B. Rifampin
- C. Griseofulvin
- D. Azithromycin
Explanation: **Explanation:** The patient is presenting with **Statin-induced Rhabdomyolysis**, characterized by the triad of muscle pain, fatigue, and dark urine (myoglobinuria). The positive dipstick for blood without RBCs on microscopy is a classic indicator of **myoglobinuria** [1]. **Why Erythromycin is correct:** Atorvastatin is metabolized by the hepatic enzyme **CYP3A4**. **Erythromycin** is a potent **CYP3A4 inhibitor**. When added to a statin regimen, it inhibits the metabolism of the statin, leading to increased plasma concentrations and an elevated risk of skeletal muscle toxicity (myopathy/rhabdomyolysis) [2]. **Analysis of Incorrect Options:** * **B. Rifampin:** This is a potent **CYP450 inducer**. It would decrease the plasma levels of atorvastatin, potentially reducing its efficacy but not causing toxicity. * **C. Griseofulvin:** This is also a **CYP450 inducer** (primarily affecting CYP1A2 and 2C9) and is not associated with precipitating statin toxicity. * **D. Azithromycin:** Unlike Erythromycin and Clarithromycin, Azithromycin is a macrolide that **does not inhibit CYP3A4**. It is generally considered safe to use with statins. **High-Yield Clinical Pearls for NEET-PG:** * **Statins to remember:** Simvastatin, Lovastatin, and Atorvastatin are CYP3A4 substrates [2]. Pravastatin and Rosuvastatin are not significantly metabolized by CYP3A4 and have a lower risk of such interactions. * **Common CYP3A4 Inhibitors (The "SICKFACES.COM" mnemonic):** **S**odium valproate, **I**soniazid, **C**imetidine, **K**etoconazole, **F**luconazole, **A**lcohol (acute), **C**hloramphenicol, **E**rythromycin/Clarithromycin, **S**ulfonamides, **C**iprofloxacin, **O**meprazole, **M**etronidazole, and **Grapefruit juice**. * **Diagnostic Clue:** Myoglobinuria (Dipstick +ve for blood, Microscopy -ve for RBCs) + Elevated Creatine Kinase (CK) = Rhabdomyolysis [2].
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