Introduction to free radicals and reactive oxygen species MCQs With Answer

Introduction to free radicals and reactive oxygen species (ROS) is essential for B. Pharm students because oxidative stress influences drug action, toxicity and disease. Free radicals are atoms or molecules with unpaired electrons, while ROS include non-radical species that readily form radicals, such as hydrogen peroxide. This overview covers sources (mitochondria, NADPH oxidase, xanthine oxidase), major ROS (superoxide, hydroxyl radical, singlet oxygen, peroxynitrite), mechanisms (Fenton and Haber–Weiss reactions), biomarkers (MDA, 8‑OHdG), and antioxidant defenses (SOD, catalase, glutathione peroxidase, vitamins C and E). Understanding redox biology, oxidative damage and antioxidant therapy is vital for pharmacology, toxicology and drug design. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the defining feature of a free radical?

• An atom or molecule with an unpaired electron
• A molecule with extra protons
• A completely stable molecule
• A molecule with a net positive charge

Correct Answer: An atom or molecule with an unpaired electron

Q2. Which of the following is NOT typically classified as a reactive oxygen species (ROS)?

• Hydrogen peroxide (H2O2)
• Superoxide anion (O2•−)
• Hydroxyl radical (•OH)
• Glucose molecule

Correct Answer: Glucose molecule

Q3. The major intracellular source of ROS under physiological conditions is:

• Mitochondrial electron transport chain
• Golgi apparatus
• Extracellular matrix
• Lysosomal hydrolases

Correct Answer: Mitochondrial electron transport chain

Q4. Which enzyme complex in phagocytes is primarily responsible for generating superoxide during the respiratory burst?

• NADPH oxidase (NOX)
• Xanthine oxidase
• Cytochrome P450
• Catalase

Correct Answer: NADPH oxidase (NOX)

Q5. The Fenton reaction involves which reactants to produce the highly reactive hydroxyl radical?

• Fe2+ and H2O2
• O2 and NADPH
• H2O and O2
• Cu2+ and superoxide

Correct Answer: Fe2+ and H2O2

Q6. The Haber–Weiss reaction overall results in formation of which species?

• Hydroxyl radical (•OH) from superoxide and hydrogen peroxide
• Hydrogen peroxide from water and oxygen
• Nitric oxide from arginine
• Singlet oxygen from ozone

Correct Answer: Hydroxyl radical (•OH) from superoxide and hydrogen peroxide

Q7. Which ROS is considered the most reactive and damaging to biomolecules?

• Hydroxyl radical (•OH)
• Hydrogen peroxide (H2O2)
• Superoxide anion (O2•−)
• Singlet oxygen (1O2)

Correct Answer: Hydroxyl radical (•OH)

Q8. Which enzyme catalyzes the dismutation of superoxide into hydrogen peroxide?

• Superoxide dismutase (SOD)
• Catalase
• Glutathione peroxidase
• Xanthine oxidase

Correct Answer: Superoxide dismutase (SOD)

Q9. Which enzyme decomposes hydrogen peroxide into water and oxygen?

• Catalase
• Glutathione reductase
• Monoamine oxidase
• Sulfotransferase

Correct Answer: Catalase

Q10. Glutathione peroxidase requires which micronutrient in its active site?

• Selenium
• Iron
• Copper
• Zinc

Correct Answer: Selenium

Q11. A decreased cellular GSH/GSSG ratio indicates:

• Increased oxidative stress
• Improved antioxidant capacity
• Decreased ROS production
• Enhanced protein synthesis

Correct Answer: Increased oxidative stress

Q12. Malondialdehyde (MDA) is a widely used biomarker for:

• Lipid peroxidation
• Protein carbonylation
• DNA methylation
• ATP depletion

Correct Answer: Lipid peroxidation

Q13. 8‑Hydroxy‑2′‑deoxyguanosine (8‑OHdG) is a marker for:

• Oxidative DNA damage
• Lipid peroxidation
• Protein nitration
• Glutathione synthesis

Correct Answer: Oxidative DNA damage

Q14. Which analytical technique is most direct for detection of free radicals in biological samples?

• Electron spin resonance (ESR) with spin trapping
• UV–Vis spectrophotometry alone
• HPLC without detectors
• Standard light microscopy

Correct Answer: Electron spin resonance (ESR) with spin trapping

Q15. Which antioxidant is lipid‑soluble and protects membranes from peroxidation?

• Vitamin E (α‑tocopherol)
• Vitamin C (ascorbic acid)
• Glutathione (GSH)
• Selenium

Correct Answer: Vitamin E (α‑tocopherol)

Q16. Which antioxidant is water‑soluble and can regenerate vitamin E radicals?

• Vitamin C (ascorbic acid)
• Vitamin K
• Uric acid only
• Beta‑carotene

Correct Answer: Vitamin C (ascorbic acid)

Q17. N‑acetylcysteine (NAC) exerts antioxidant effects primarily by:

• Serving as a precursor for glutathione synthesis
• Directly chelating DNA
• Inhibiting catalase irreversibly
• Generating superoxide

Correct Answer: Serving as a precursor for glutathione synthesis

Q18. Which herbicide is known to undergo redox cycling and produce superoxide, causing pulmonary toxicity?

• Paraquat
• Glyphosate
• Atrazine
• 2,4‑Dichlorophenoxyacetic acid

Correct Answer: Paraquat

Q19. Peroxynitrite (ONOO−) is formed by the reaction of:

• Superoxide (O2•−) with nitric oxide (NO)
• Hydrogen peroxide with chloride
• Nitrite with hydrogen sulfide
• Singlet oxygen with water

Correct Answer: Superoxide (O2•−) with nitric oxide (NO)

Q20. The oxidative burst in neutrophils primarily depends on which substrate for NADPH oxidase?

• NADPH
• NADH
• ATP
• GTP

Correct Answer: NADPH

Q21. Which reaction is the primary non‑enzymatic source of hydroxyl radicals in biological systems?

• Fenton reaction (Fe2+ + H2O2 → Fe3+ + •OH + OH−)
• Disproportionation of nitric oxide
• Dehydrogenation of fatty acids
• Reduction of oxygen by catalase

Correct Answer: Fenton reaction (Fe2+ + H2O2 → Fe3+ + •OH + OH−)

Q22. Which assay estimates total antioxidant power by reducing ferric (Fe3+) to ferrous (Fe2+) ions?

• FRAP (Ferric Reducing Antioxidant Power)
• DPPH radical scavenging assay
• TUNEL assay
• Comet assay

Correct Answer: FRAP (Ferric Reducing Antioxidant Power)

Q23. Which enzyme uses NADPH to regenerate reduced glutathione (GSH) from oxidized glutathione (GSSG)?

• Glutathione reductase
• Glutathione peroxidase
• Catalase
• Superoxide dismutase

Correct Answer: Glutathione reductase

Q24. Chronic oxidative stress is strongly implicated in the pathogenesis of which condition?

• Atherosclerosis
• Type O blood phenotype
• Acute fracture healing only
• Hair color change exclusively

Correct Answer: Atherosclerosis

Q25. Which statement about hydrogen peroxide (H2O2) is TRUE?

• It is not a radical, can diffuse across membranes, and can form •OH via Fenton chemistry
• It is the most reactive radical and cannot cross membranes
• It is produced only by peroxisomes and has no signaling role
• It irreversibly destroys antioxidant enzymes on contact

Correct Answer: It is not a radical, can diffuse across membranes, and can form •OH via Fenton chemistry

Q26. Which molecule can regenerate oxidized vitamin E back to its reduced, active form?

• Vitamin C (ascorbate)
• Vitamin D
• Iron
• Urea

Correct Answer: Vitamin C (ascorbate)

Q27. The TBARS assay is commonly used to estimate levels of which compound produced during lipid peroxidation?

• Malondialdehyde (MDA)
• 8‑OHdG
• Protein carbonyls
• Glutathione

Correct Answer: Malondialdehyde (MDA)

Q28. Which SOD isoform is localized predominantly in mitochondria?

• SOD2 (Mn‑SOD)
• SOD1 (Cu/Zn‑SOD)
• SOD3 (extracellular SOD)
• All SOD isoforms are exclusively nuclear

Correct Answer: SOD2 (Mn‑SOD)

Q29. Which nitric oxide synthase (NOS) isoform is classically inducible and involved in inflammatory ROS/RNS production?

• iNOS (inducible NOS)
• eNOS (endothelial NOS)
• nNOS (neuronal NOS)
• mNOS (mitochondrial NOS, not inducible)

Correct Answer: iNOS (inducible NOS)

Q30. Which clinical antidote acts by replenishing glutathione stores and is used in acetaminophen overdose?

• N‑acetylcysteine (NAC)
• Vitamin K
• Deferoxamine
• Atropine

Correct Answer: N‑acetylcysteine (NAC)

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