Oxidation MCQs With Answer provide B.Pharm students with focused practice on oxidative processes relevant to pharmaceuticals, drug metabolism and formulation stability. This set covers key concepts such as redox reactions, oxidation states, cytochrome P450-mediated phase I metabolism, reactive oxygen species, free-radical chain mechanisms, antioxidant protection, metal-catalyzed degradation and analytical assays for peroxides. Emphasis is on mechanisms, enzyme cofactors, oxidative degradation pathways of APIs and strategies to prevent oxidation in formulations. Designed to deepen understanding for exams and practical formulation work, these MCQs combine theoretical and applied questions to strengthen problem-solving skills in pharmaceutical oxidation. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the most general definition of oxidation in the context of drug chemistry?
- Gain of electrons by a molecule
- Loss of electrons or increase in oxidation state
- Formation of hydrogen bonds
- Increase in molecular weight only
Correct Answer: Loss of electrons or increase in oxidation state
Q2. Which pair commonly acts as oxidizing agents in pharmaceutical formulations and can initiate API degradation?
- Molecular oxygen and peroxides
- Nitrogen and argon
- Sodium chloride and potassium chloride
- Water and ethanol
Correct Answer: Molecular oxygen and peroxides
Q3. Which enzyme family is primarily responsible for oxidative phase I metabolism of many drugs in the liver?
- Glutathione S-transferases
- Cytochrome P450 monooxygenases
- UDP-glucuronosyltransferases
- Acetyltransferases
Correct Answer: Cytochrome P450 monooxygenases
Q4. Which cofactor provides reducing equivalents to cytochrome P450 enzymes for oxidative metabolism?
- ATP
- FADH2
- NADPH
- Coenzyme A
Correct Answer: NADPH
Q5. Oxidation reactions in drug metabolism are classified under which phase of biotransformation?
- Phase III transport
- Phase II conjugation
- Phase I functionalization
- Phase IV elimination
Correct Answer: Phase I functionalization
Q6. Which analytical method is commonly used to quantify peroxides formed during oxidative degradation of lipid-containing formulations?
- UV-visible spectroscopy at 280 nm
- Iodometric titration (peroxide value)
- Gas chromatography with nitrogen detector
- pH titration
Correct Answer: Iodometric titration (peroxide value)
Q7. Autoxidation of susceptible drug molecules typically proceeds by which mechanistic pathway?
- Nucleophilic substitution
- Free-radical chain reaction (initiation, propagation, termination)
- Concerted pericyclic rearrangement
- Acid-catalyzed hydrolysis
Correct Answer: Free-radical chain reaction (initiation, propagation, termination)
Q8. Primary (chain-breaking) antioxidants prevent propagation by doing what?
- Chelating metal ions only
- Donating a hydrogen atom to peroxyl radicals
- Increasing solution pH
- Forming insoluble precipitates
Correct Answer: Donating a hydrogen atom to peroxyl radicals
Q9. Which compound is a classic lipophilic chain-breaking antioxidant used to protect lipidic formulations and membranes?
- Ascorbic acid
- Alpha-tocopherol (Vitamin E)
- Sodium metabisulfite
- EDTA
Correct Answer: Alpha-tocopherol (Vitamin E)
Q10. Which metal ion commonly catalyzes oxidation via Fenton-type chemistry, producing highly reactive hydroxyl radicals?
- Calcium (Ca2+)
- Iron (Fe2+)
- Magnesium (Mg2+)
- Potassium (K+)
Correct Answer: Iron (Fe2+)
Q11. Oxidative stability testing of pharmaceuticals typically accelerates oxidation using which combination of factors?
- Low temperature, darkness, and vacuum
- Elevated temperature, light exposure and oxygen or peroxide stress
- pH 7 buffer only
- Freezing and lyophilization
Correct Answer: Elevated temperature, light exposure and oxygen or peroxide stress
Q12. Which packaging strategy is widely used to reduce oxygen-mediated oxidation during manufacturing and storage?
- Adding sucrose to formulations
- Nitrogen purging (inert atmosphere) and oxygen scavengers
- Using transparent glass to allow light
- Storing at high humidity
Correct Answer: Nitrogen purging (inert atmosphere) and oxygen scavengers
Q13. Which structural motif in APIs is particularly susceptible to oxidation to form carbonyl products?
- Aromatic nitro groups
- Benzylic methylene (benzylic oxidation)
- Quaternary ammonium centers
- Triazole rings
Correct Answer: Benzylic methylene (benzylic oxidation)
Q14. Which of the following represents oxidation of an iron ion?
- Fe3+ → Fe2+
- Fe2+ → Fe3+
- Fe0 → Fe-1
- Fe2+ → Fe1+
Correct Answer: Fe2+ → Fe3+
Q15. What is the most suitable technique to directly detect short-lived free radicals formed during drug autoxidation?
- Infrared spectroscopy
- Electron spin resonance (ESR) spectroscopy
- High-performance liquid chromatography (HPLC)
- Mass spectrometry without spin trap
Correct Answer: Electron spin resonance (ESR) spectroscopy
Q16. Besides cytochrome P450, which enzyme catalyzes oxidative deamination of monoamine drugs and endogenous amines?
- Monoamine oxidase (MAO)
- Alcohol dehydrogenase
- Cyclooxygenase
- Thioltransferase
Correct Answer: Monoamine oxidase (MAO)
Q17. What is the primary role of reduced glutathione (GSH) in protecting cells and formulations from oxidative damage?
- Acting as a surfactant
- Serving as a reducing agent and substrate for glutathione peroxidase to detoxify peroxides
- Raising pH to basic levels
- Chelating lipids to prevent oxidation
Correct Answer: Serving as a reducing agent and substrate for glutathione peroxidase to detoxify peroxides
Q18. Lipid peroxidation yields malondialdehyde (MDA); which assay is commonly used to estimate MDA as a marker of oxidative damage?
- TBARS (thiobarbituric acid reactive substances) assay
- BCA protein assay
- Kjeldahl nitrogen analysis
- Bradford assay
Correct Answer: TBARS (thiobarbituric acid reactive substances) assay
Q19. Photooxidation of drugs in the presence of photosensitizers commonly proceeds via formation of which reactive species?
- Singlet oxygen (1O2)
- Dinitrogen gas (N2)
- Hydrochloric acid (HCl)
- Ozone (O3)
Correct Answer: Singlet oxygen (1O2)
Q20. Which combination is an effective formulation approach to reduce photooxidative degradation?
- Use of UV absorbers, antioxidants and opaque packaging
- Increasing dissolved oxygen and using clear bottles
- Adding iron salts to catalyze oxidation
- Raising formulation pH to extreme acidic values only
Correct Answer: Use of UV absorbers, antioxidants and opaque packaging
Q21. How does enzymatic oxidation differ from nonenzymatic oxidation in pharmaceutical systems?
- Enzymatic oxidation is non-specific and random
- Enzymatic oxidation is substrate-specific and catalyzed under physiological conditions
- Nonenzymatic oxidation requires enzymes
- There is no difference; both require ATP
Correct Answer: Enzymatic oxidation is substrate-specific and catalyzed under physiological conditions
Q22. Cytochrome P450 monooxygenases commonly perform which type of chemical transformation on substrates?
- Hydrolysis of esters only
- Monooxygenation, inserting one oxygen atom into the substrate
- Direct methylation of aromatic rings
- Cleavage of disulfide bonds
Correct Answer: Monooxygenation, inserting one oxygen atom into the substrate
Q23. Which of the following are typical reactive oxygen species (ROS) relevant to drug oxidation and oxidative stress?
- Superoxide (O2-), hydrogen peroxide (H2O2) and hydroxyl radical (OH·)
- Nitrogen gas, methane and ethane
- Sodium, potassium and chloride ions
- Carbon dioxide, nitrogen dioxide, and helium
Correct Answer: Superoxide (O2-), hydrogen peroxide (H2O2) and hydroxyl radical (OH·)
Q24. Oxidative degradation kinetics of many drugs under constant excess oxygen often follow which rate law?
- Zero-order kinetics only
- Second-order kinetics with respect to humidity
- Pseudo-first-order kinetics with respect to the drug
- Non-kinetic instantaneous decay
Correct Answer: Pseudo-first-order kinetics with respect to the drug
Q25. Which excipient is commonly used as a metal chelator to inhibit metal-catalyzed oxidation in formulations?
- EDTA (ethylenediaminetetraacetic acid)
- Polyethylene glycol (PEG)
- Sucrose
- Starch
Correct Answer: EDTA (ethylenediaminetetraacetic acid)
Q26. Which commonly used polymeric excipient can form peroxides on storage and thus act as a source of oxidants?
- Polyethylene glycol (PEG)
- Microcrystalline cellulose
- Calcium phosphate
- Lactose monohydrate
Correct Answer: Polyethylene glycol (PEG)
Q27. For an aqueous formulation, which antioxidant is most appropriate due to its water solubility?
- Alpha-tocopherol
- Ascorbic acid (vitamin C)
- BHT (butylated hydroxytoluene)
- Propyl gallate (lipophilic form)
Correct Answer: Ascorbic acid (vitamin C)
Q28. Oxidation of protein therapeutics commonly affects which amino acid functional group, leading to activity loss?
- Oxidation of thiol (-SH) groups to form disulfides or sulfenic/sulfinic acids
- Phosphorylation of serine residues
- Methylation of lysine residues
- Glycosylation of asparagine residues
Correct Answer: Oxidation of thiol (-SH) groups to form disulfides or sulfenic/sulfinic acids
Q29. A redox couple with a more positive standard reduction potential is more likely to:
- Lose electrons and act as a reducing agent
- Be oxidized preferentially
- Gain electrons and act as an oxidizing agent
- Be inert and neither oxidize nor reduce
Correct Answer: Gain electrons and act as an oxidizing agent
Q30. How does lyophilization (freeze-drying) generally influence oxidative degradation of solid-dosage biologicals?
- Increases oxidation by concentrating oxygen in the matrix
- Has no effect on oxidation
- Reduces oxidation by removing water and decreasing molecular mobility and oxygen accessibility
- Converts all thiols to oxidized forms during the process
Correct Answer: Reduces oxidation by removing water and decreasing molecular mobility and oxygen accessibility
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