Introduction: Drug metabolism is a core topic in pharmacokinetics that examines how the body chemically modifies drugs through biotransformation. B. Pharm students must understand hepatic and extrahepatic metabolism, Phase I (oxidation, reduction, hydrolysis) and Phase II (conjugation) reactions, roles of CYP450 enzymes, factors affecting clearance and half-life, first-pass effect, prodrugs and bioactivation, and clinically important drug–drug interactions. Key concepts include intrinsic clearance, extraction ratio, enzyme induction and inhibition, genetic polymorphisms (e.g., CYP2D6, NAT2), and transporter influence (P-gp). Mastery of these mechanisms supports rational dosing, safety, and therapeutic optimization. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the primary purpose of Phase I metabolic reactions in drug biotransformation?
- Increase molecular polarity directly for excretion
- Introduce or expose functional groups to facilitate Phase II conjugation
- Attach glucuronic acid to the drug molecule
- Transport drug across cell membranes
Correct Answer: Introduce or expose functional groups to facilitate Phase II conjugation
Q2. Which cytochrome P450 isoform is most commonly implicated in the metabolism of a wide range of clinically used drugs?
- CYP1A2
- CYP2D6
- CYP3A4
- CYP2C19
Correct Answer: CYP3A4
Q3. Glucuronidation is catalyzed by which enzyme family?
- Sulfotransferases (SULTs)
- N-acetyltransferases (NATs)
- UDP-glucuronosyltransferases (UGTs)
- Glutathione S-transferases (GSTs)
Correct Answer: UDP-glucuronosyltransferases (UGTs)
Q4. First-pass metabolism primarily affects which route of drug administration?
- Intravenous
- Intramuscular
- Oral
- Subcutaneous
Correct Answer: Oral
Q5. Which of the following is an example of a Phase II conjugation that increases water solubility?
- Oxidation by CYP450
- Hydrolysis by esterases
- Glucuronidation
- Reduction by nitroreductases
Correct Answer: Glucuronidation
Q6. A drug exhibiting zero-order kinetics means:
- Rate of elimination is proportional to drug concentration
- Rate of elimination is constant regardless of concentration
- Drug is eliminated only by renal filtration
- Drug follows Michaelis-Menten kinetics with high Km
Correct Answer: Rate of elimination is constant regardless of concentration
Q7. Which phase I reaction is primarily responsible for oxidative metabolism of many lipophilic drugs?
- Conjugation with sulfate
- CYP450-mediated hydroxylation
- Acetylation
- Glutathione conjugation
Correct Answer: CYP450-mediated hydroxylation
Q8. Which statement best defines intrinsic clearance (Clint)?
- Clearance limited only by renal blood flow
- Ability of the liver to remove drug in absence of flow limitations and binding constraints
- Volume of plasma completely cleared per unit time in the whole body
- Clearance after oral dosing only
Correct Answer: Ability of the liver to remove drug in absence of flow limitations and binding constraints
Q9. Which enzyme group is responsible for acetylation of drugs such as isoniazid and sulfonamides?
- UDP-glucuronosyltransferases
- N-acetyltransferases (NATs)
- Sulfotransferases (SULTs)
- Monoamine oxidases (MAOs)
Correct Answer: N-acetyltransferases (NATs)
Q10. Which process can lead to drug bioactivation producing a toxic metabolite?
- Conjugation to glucuronic acid
- Hydrolysis to inactive polar metabolites
- CYP-mediated oxidation producing a reactive intermediate
- Sulfation that always increases excretion
Correct Answer: CYP-mediated oxidation producing a reactive intermediate
Q11. Which transport protein is most associated with efflux of drugs from intestinal epithelium affecting oral bioavailability?
- Albumin
- P-glycoprotein (P-gp)
- OATP1B1
- Renal organic anion transporter (OAT)
Correct Answer: P-glycoprotein (P-gp)
Q12. Competitive inhibition of a drug-metabolizing enzyme typically results in:
- Permanent enzyme inactivation requiring new enzyme synthesis
- No change in Km and decreased Vmax
- Increase in apparent Km with unchanged Vmax
- Decrease in both Km and Vmax
Correct Answer: Increase in apparent Km with unchanged Vmax
Q13. Enterohepatic recirculation affects drug pharmacokinetics by:
- Decreasing drug half-life by rapid renal excretion
- Causing secondary peaks and prolonging half-life
- Preventing conjugation reactions in the liver
- Increasing protein binding in plasma
Correct Answer: Causing secondary peaks and prolonging half-life
Q14. Which conjugation pathway becomes saturated at high substrate concentrations and is important for endogenous and xenobiotic metabolism?
- Glucuronidation
- Sulfation
- Acetylation
- Glutathione conjugation
Correct Answer: Sulfation
Q15. A potent CYP3A4 inducer like rifampin is expected to:
- Decrease metabolism of CYP3A4 substrates leading to toxicity
- Increase metabolism of CYP3A4 substrates leading to reduced plasma levels
- Block P-gp efflux and increase absorption
- Directly inhibit renal clearance mechanisms
Correct Answer: Increase metabolism of CYP3A4 substrates leading to reduced plasma levels
Q16. Genetic polymorphism in CYP2D6 can result in which clinical phenotypes?
- Only normal metabolizers for all drugs
- Poor, intermediate, extensive, and ultra-rapid metabolizers
- Universal protection against drug toxicity
- Elimination of phase II reactions
Correct Answer: Poor, intermediate, extensive, and ultra-rapid metabolizers
Q17. Which type of metabolic reaction is primarily catalyzed by esterases?
- Oxidation
- Reduction
- Hydrolysis of ester or amide bonds
- Glutathione conjugation
Correct Answer: Hydrolysis of ester or amide bonds
Q18. Hepatic extraction ratio is defined as:
- Fraction of drug removed by kidney per pass through liver
- Fraction of drug removed by liver during one pass through hepatic circulation
- Ratio of bile excretion to urine excretion
- Fraction of drug bound to plasma proteins
Correct Answer: Fraction of drug removed by liver during one pass through hepatic circulation
Q19. Which conjugation reaction uses glutathione to detoxify reactive metabolites?
- Glucuronidation
- Sulfation
- Glutathione S-transferase-mediated conjugation
- Acetylation by NAT
Correct Answer: Glutathione S-transferase-mediated conjugation
Q20. In Michaelis-Menten kinetics, Vmax represents:
- The substrate concentration at which velocity is half maximal
- The maximum reaction velocity when the enzyme is saturated with substrate
- The enzyme affinity for substrate
- The clearance of the drug from plasma
Correct Answer: The maximum reaction velocity when the enzyme is saturated with substrate
Q21. Which factor decreases hepatic clearance of a drug with low extraction ratio?
- Decreased plasma protein binding
- Reduced hepatic enzyme activity
- Increased hepatic blood flow
- Enhanced first-pass effect
Correct Answer: Reduced hepatic enzyme activity
Q22. Prodrugs are designed to:
- Be pharmacologically active without metabolism
- Enhance absorption or targeting by undergoing metabolic activation
- Always avoid hepatic metabolism entirely
- Reduce the need for phase II reactions
Correct Answer: Enhance absorption or targeting by undergoing metabolic activation
Q23. Which of the following is a non-microsomal metabolic pathway?
- CYP450-mediated oxidation in the endoplasmic reticulum
- Monoamine oxidase activity in mitochondria
- UGT-mediated glucuronidation in microsomes
- Sulfotransferase activity in the cytosol
Correct Answer: Monoamine oxidase activity in mitochondria
Q24. A drug with high protein binding is likely to have which characteristic regarding hepatic clearance?
- High free fraction and high clearance
- Low free fraction and potentially reduced clearance for low extraction drugs
- No change in clearance with enzyme inhibition
- Always rapid renal elimination
Correct Answer: Low free fraction and potentially reduced clearance for low extraction drugs
Q25. Which agent is a known CYP450 inhibitor that can increase plasma levels of coadministered substrates?
- Rifampin
- Phenobarbital
- Ketoconazole
- Carbamazepine
Correct Answer: Ketoconazole
Q26. Sulfation uses which cofactor for conjugation reactions?
- UDP-glucuronic acid
- PAPS (3′-phosphoadenosine-5′-phosphosulfate)
- Acetyl-CoA
- Glutathione
Correct Answer: PAPS (3′-phosphoadenosine-5′-phosphosulfate)
Q27. Which metabolic pathway is particularly important for detoxification of electrophilic intermediates and protection against hepatotoxicity?
- Acetylation by NAT
- Glutathione conjugation
- Phase I reduction only
- Sulfation at high capacity
Correct Answer: Glutathione conjugation
Q28. Renal excretion of a metabolite is increased when:
- Metabolite is more lipophilic than parent drug
- Metabolite is highly protein bound
- Metabolite is more hydrophilic and polar than parent drug
- Metabolite undergoes enterohepatic recycling
Correct Answer: Metabolite is more hydrophilic and polar than parent drug
Q29. Which process can reduce first-pass metabolism and increase oral bioavailability?
- Co-administration with a CYP inducer
- Use of a prodrug activated in the liver
- Formulation for sublingual or buccal absorption
- Increasing hepatic blood flow
Correct Answer: Formulation for sublingual or buccal absorption
Q30. Which parameter describes the time required for plasma drug concentration to decrease by 50%?
- Clearance
- Volume of distribution
- Half-life (t1/2)
- Bioavailability
Correct Answer: Half-life (t1/2)
