Drug Metabolism Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for Drug Metabolism. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Drug Metabolism Indian Medical PG Question 1: Which antiepileptic drug requires regular monitoring of liver enzymes?
- A. Valproate (Correct Answer)
- B. Levetiracetam
- C. Lamotrigine
- D. Phenytoin
Drug Metabolism Explanation: ***Valproate***
- **Valproate** is known to cause dose-related **hepatotoxicity**, ranging from asymptomatic enzyme elevation to fatal hepatic failure, especially in young children and those on polytherapy.
- Regular monitoring of **liver function tests (LFTs)** is crucial to detect early signs of liver injury and adjust treatment.
*Levetiracetam*
- **Levetiracetam** is generally well-tolerated and does not typically require routine monitoring of liver enzymes due to its primary renal excretion and low potential for hepatotoxicity.
- While mild, transient elevations in LFTs can occur, they are usually not clinically significant.
*Lamotrigine*
- **Lamotrigine** is primarily known for causing serious **skin rashes** (e.g., Stevens-Johnson syndrome) rather than significant hepatotoxicity.
- Routine liver enzyme monitoring is generally not recommended unless there are other risk factors for hepatic dysfunction.
*Phenytoin*
- Although **phenytoin** can cause idiosyncratic liver injury, it is not as consistently associated with dose-related hepatotoxicity as valproate.
- Monitoring of **phenytoin levels** is crucial [1], but routine liver enzyme monitoring is not typically mandated as strictly as for valproate.
Drug Metabolism Indian Medical PG Question 2: 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
Drug Metabolism 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.
Drug Metabolism Indian Medical PG Question 3: A factor that is likely to increase the duration of action of a drug that is partially metabolized by CYP3A4 in the liver is:
- A. Chronic administration of phenobarbital with the drug
- B. Displacement from tissue binding sites by another drug
- C. Chronic administration of rifampicin
- D. Chronic administration of cimetidine with the drug (Correct Answer)
Drug Metabolism Explanation: ***Chronic administration of cimetidine with the drug***
- **Cimetidine** is a potent inhibitor of various **cytochrome P450 (CYP450) enzymes**, including **CYP3A4**.
- By inhibiting the metabolism of a drug predominantly metabolized by **CYP3A4**, cimetidine will increase its plasma concentration and extend its **duration of action**.
*Chronic administration of phenobarbital with the drug*
- **Phenobarbital** is a strong **inducer of CYP450 enzymes**, including **CYP3A4**.
- Induction would accelerate the metabolism of the drug, thus **decreasing its duration of action**, not increasing it.
*Displacement from tissue binding sites by another drug*
- Displacement from tissue binding sites would primarily increase the **free fraction of the drug in the plasma**, leading to a more rapid distribution to eliminating organs and potentially **shorter duration of action** if elimination is extraction-limited.
- This mechanism does not directly impact the **metabolic rate** unless clearance is significantly altered through increased availability for metabolism.
*Chronic administration of rifampicin*
- **Rifampicin** is a potent **inducer of CYP3A4** and other CYP enzymes.
- Its administration would lead to **increased metabolism** of the co-administered drug, thereby **reducing its duration of action**.
Drug Metabolism Indian Medical PG Question 4: Phase 1 biotransformation includes
- A. Reduction (Correct Answer)
- B. Methylation
- C. Acetylation
- D. Sulfate conjugation
Drug Metabolism Explanation: ***Reduction***
- **Phase 1 biotransformation reactions** are non-synthetic reactions that introduce or expose polar functional groups (e.g., -OH, -NH2, -SH) on xenobiotics to make them more water-soluble.
- The three main Phase 1 reactions are **oxidation**, **reduction**, and **hydrolysis**.
- These reactions typically involve **cytochrome P450 enzymes** and prepare drugs for excretion or Phase 2 conjugation.
*Acetylation*
- **Acetylation** is a **Phase 2 (conjugation) reaction**, not Phase 1.
- Involves transfer of an acetyl group to amine-containing drugs via **N-acetyltransferase**.
- Increases water solubility and facilitates excretion.
*Sulfate conjugation*
- **Sulfate conjugation** is a **Phase 2 (conjugation) reaction**, not Phase 1.
- Involves addition of a sulfate group via **sulfotransferase enzymes**.
- Significantly increases hydrophilicity for renal excretion.
*Methylation*
- **Methylation** is a **Phase 2 (conjugation) reaction**, not Phase 1.
- Involves addition of a methyl group via **methyltransferase enzymes**.
- Unlike most Phase 2 reactions, methylation may sometimes **decrease** water solubility but is still classified as conjugation.
Drug Metabolism Indian Medical PG Question 5: Administration of which of the following drugs would increase the bioavailability of saquinavir?
- A. Cimetidine
- B. Vitamin C
- C. Ritonavir (Correct Answer)
- D. Ganciclovir
Drug Metabolism Explanation: **Ritonavir**
- **Ritonavir** is a potent **CYP3A4 inhibitor**, which is the primary enzyme responsible for the metabolism of saquinavir.
- By inhibiting **saquinavir** metabolism, ritonavir significantly **increases its plasma concentrations and bioavailability**, making it an effective pharmacokinetic enhancer.
- This combination (saquinavir/ritonavir) is a clinically established strategy in antiretroviral therapy.
*Cimetidine*
- **Cimetidine** inhibits various cytochrome P450 enzymes but is a less potent and more general inhibitor compared to ritonavir, particularly for **CYP3A4**.
- While it could theoretically have some effect on drug metabolism, its impact on saquinavir's bioavailability would be **clinically insignificant** compared to ritonavir.
*Vitamin C*
- **Vitamin C** (ascorbic acid) is an antioxidant and plays various roles in the body.
- It has **no significant interaction** with cytochrome P450 enzymes and would not affect the metabolism or bioavailability of saquinavir.
*Ganciclovir*
- **Ganciclovir** is an antiviral drug primarily used to treat cytomegalovirus (CMV) infections.
- It does not significantly inhibit or induce cytochrome P450 enzymes and would therefore **not affect the bioavailability** of saquinavir.
Drug Metabolism Indian Medical PG Question 6: Glucuronide reaction is seen in
- A. Phase 2 (Correct Answer)
- B. NADPH-dependent reaction
- C. Phase 1
- D. Non enzymatic reaction
Drug Metabolism Explanation: ***Phase 2***
- **Glucuronide conjugation** is a prominent **Phase 2 biotransformation reaction** where glucuronic acid is added to a drug or metabolite.
- This reaction increases the **water solubility** of xenobiotics, facilitating their excretion from the body.
- Catalyzed by **UDP-glucuronosyltransferase (UGT)** enzymes using **UDP-glucuronic acid** as the donor molecule.
*NADPH-dependent reaction*
- **Glucuronidation does not require NADPH** as a cofactor.
- **NADPH** is primarily involved in **Phase 1 reactions** catalyzed by cytochrome P450 enzymes for oxidation and reduction reactions.
- The glucuronidation reaction uses **UDP-glucuronic acid**, not NADPH, as the source of the glucuronic acid moiety.
*Phase 1*
- **Phase 1 reactions** typically involve **oxidation**, **reduction**, or **hydrolysis**, introducing or unmasking functional groups (e.g., -OH, -SH, -NH2).
- These reactions aim to make the parent compound more polar and often serve as a prelude to Phase 2 reactions.
- Glucuronidation is a Phase 2 conjugation reaction, not Phase 1.
*Non enzymatic reaction*
- **Glucuronidation** is a highly specific **enzymatic reaction** catalyzed by UDP-glucuronosyltransferase (UGT) enzymes.
- **Non-enzymatic reactions** in drug metabolism are less common and typically involve spontaneous degradation or chemical rearrangements without enzyme involvement.
Drug Metabolism Indian Medical PG Question 7: A patient is on warfarin therapy. All of the following drugs increase the risk of bleeding with warfarin except?
- A. Isoniazid
- B. Amiodarone
- C. Carbamazepine (Correct Answer)
- D. Cimetidine
Drug Metabolism Explanation: ***Carbamazepine***
- Carbamazepine **induces cytochrome P450 enzymes**, specifically **CYP3A4** and **CYP2C9**, which are responsible for warfarin metabolism.
- This induction leads to a **faster metabolism of warfarin**, thus **decreasing its anticoagulant effect** and thereby reducing the risk of bleeding.
*Isoniazid*
- Isoniazid is an **inhibitor of cytochrome P450 enzymes**, primarily **CYP2C9**, which metabolizes the more potent S-warfarin isomer.
- This inhibition **decreases warfarin metabolism**, leading to **increased anticoagulant effect** and higher risk of bleeding.
*Amiodarone*
- Amiodarone is a potent **inhibitor of cytochrome P450 enzymes**, significantly **CYP2C9** and **CYP3A4**.
- It leads to a **reduced metabolism of warfarin**, causing **elevated INR** and an increased risk of bleeding.
*Cimetidine*
- Cimetidine is a known **inhibitor of various cytochrome P450 enzymes**, particularly **CYP1A2**, **CYP2C9**, and **CYP3A4**.
- Its inhibitory action on warfarin metabolism results in **higher warfarin levels** and an **increased risk of bleeding**.
Drug Metabolism Indian Medical PG Question 8: Bile salts undergo conjugation for enhanced solubility:
- A. After conjugation with derived proteins
- B. After conjugation with lysine
- C. After conjugation with taurine and glycine (Correct Answer)
- D. After conjugation with betaglucuronic acid
Drug Metabolism Explanation: ***After conjugation with taurine and glycine***
- This statement accurately describes the most common conjugation pathway for bile acids, increasing their **amphipathic properties** and solubility.
- Conjugation with these amino acids forms **bile salts** (e.g., glycocholate, taurocholate), which are essential for **micelle formation** and fat digestion.
- This is the primary mechanism by which bile acids become bile salts with enhanced solubility.
*After conjugation with betaglucuronic acid*
- While bile acids do undergo conjugation for increased solubility, they are primarily conjugated with glycine or taurine, not beta-glucuronic acid.
- Conjugation with beta-glucuronic acid is a common detoxification pathway for many xenobiotics and bilirubin, but not the primary method for bile acids.
*After conjugation with derived proteins*
- Bile salts are primarily steroid derivatives and are not conjugated with derived proteins.
- The purpose of conjugation is to increase hydrophilicity, which proteins would not achieve in this context.
*After conjugation with lysine*
- Lysine is an amino acid but is not involved in the conjugation of bile acids.
- Bile acid conjugation specifically uses the amino acids glycine and taurine.
Drug Metabolism Indian Medical PG Question 9: All of the following are true about glutathione, except:
- A. It is co-factor of various enzymes
- B. It converts hemoglobin to methemoglobin (Correct Answer)
- C. It is a tripeptide
- D. It conjugates xenobiotics
Drug Metabolism Explanation: ***It converts hemoglobin to methemoglobin***
- Glutathione is a **reducing agent** that helps protect hemoglobin from oxidation, thus **preventing** the formation of methemoglobin.
- **Methemoglobin** occurs when the iron in hemoglobin is oxidized from the ferrous (Fe2+) to the ferric (Fe3+) state, which is a process glutathione actively counters.
*It is co-factor of various enzymes*
- Glutathione serves as a crucial **co-factor** for several enzymes, including **glutathione peroxidase**, which plays a vital role in antioxidant defense.
- It participates in various **detoxification reactions** and catalyzes the reduction of harmful reactive oxygen species.
*It is a tripeptide*
- Glutathione is indeed a **tripeptide** composed of three amino acids: **glutamate**, **cysteine**, and **glycine**.
- Its unique structure enables its diverse biological functions, including its prominent role as an antioxidant.
*It conjugates xenobiotics*
- Glutathione plays a critical role in **detoxifying xenobiotics** (foreign compounds) by conjugating with them, making them more water-soluble and easier to excrete.
- This process is mediated by **glutathione S-transferases**, which attach glutathione to various toxic compounds.
Drug Metabolism Indian Medical PG Question 10: In the liver, what is ethanol primarily converted to?
- A. Methanol
- B. Pyruvate
- C. Acetaldehyde (Correct Answer)
- D. Oxaloacetate
Drug Metabolism Explanation: **Explanation:**
The metabolism of ethanol primarily occurs in the liver through a series of oxidative reactions. The first and rate-limiting step involves the conversion of **ethanol to acetaldehyde**. This reaction is catalyzed by the cytosolic enzyme **Alcohol Dehydrogenase (ADH)**, which utilizes $NAD^+$ as a co-factor, reducing it to $NADH$. Acetaldehyde is a highly reactive and toxic intermediate responsible for many of the adverse effects of alcohol consumption (e.g., nausea, tachycardia). It is subsequently converted to acetate by Mitochondrial Aldehyde Dehydrogenase (ALDH2).
**Analysis of Incorrect Options:**
* **Methanol (A):** Methanol is a different type of alcohol (wood alcohol). It is not a metabolite of ethanol; rather, it is metabolized by the same enzyme system into toxic formaldehyde and formic acid.
* **Pyruvate (B):** Pyruvate is the end-product of glycolysis. While ethanol metabolism increases the $NADH/NAD^+$ ratio, this actually shifts the equilibrium *away* from pyruvate, converting it into lactate instead (leading to lactic acidosis).
* **Oxaloacetate (D):** Oxaloacetate is an intermediate of the TCA cycle and gluconeogenesis. High levels of $NADH$ from ethanol metabolism cause oxaloacetate to be diverted to malate, contributing to the inhibition of gluconeogenesis and subsequent fasting hypoglycemia.
**High-Yield Clinical Pearls for NEET-PG:**
1. **Disulfiram (Antabuse):** Inhibits **Aldehyde Dehydrogenase**, causing acetaldehyde accumulation. This leads to the "Disulfiram-like reaction" (flushing, vomiting), used as a deterrent in chronic alcoholism.
2. **Fomepizole:** Inhibits **Alcohol Dehydrogenase**; it is the preferred antidote for methanol or ethylene glycol poisoning.
3. **Metabolic Derangements:** Ethanol metabolism increases the $NADH/NAD^+$ ratio, leading to hypoglycemia, lactic acidosis, and fatty liver (steatosis) due to increased fatty acid synthesis.
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