Vitamin C, Vitamin E, α-Lipoic acid and Melatonin MCQs With Answer

Introduction: Vitamin C, Vitamin E, α-Lipoic acid and Melatonin are key antioxidants and modulators studied in pharmaceutical sciences. This review-focused MCQ set covers chemistry, mechanisms of antioxidant action, redox cycling, absorption, metabolism, clinical uses (eg, scurvy prevention, membrane protection, diabetic neuropathy, circadian disorders), formulation and stability, drug interactions, adverse effects, and evidence-based therapeutic roles. Emphasis is placed on pharmacokinetics, enzymatic cofactors, receptor pharmacology and clinically relevant precautions for B. Pharm students. Questions probe deeper concepts such as pro-oxidant potential, hepatic CYP interactions, stereochemistry, and clinical dosing considerations. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which of the following best describes the primary antioxidant mechanism of vitamin C?

• Acting as a metal chelator only
• Donating electrons to neutralize free radicals and regenerating other antioxidants
• Serving exclusively as a membrane lipid stabilizer
• Inhibiting cytochrome P450 enzymes to prevent oxidation

Correct Answer: Donating electrons to neutralize free radicals and regenerating other antioxidants

Q2. Vitamin C is an essential cofactor for which enzyme family important in collagen synthesis?

• Cytochrome P450 oxidases
• Prolyl and lysyl hydroxylases
• Glutathione peroxidases
• Monoamine oxidases

Correct Answer: Prolyl and lysyl hydroxylases

Q3. Which oxidized form of vitamin C is transported into cells via GLUT transporters?

• Ascorbic acid
• Ascorbyl palmitate
• Dehydroascorbic acid
• Ascorbate radical

Correct Answer: Dehydroascorbic acid

Q4. High-dose intravenous vitamin C may cause hemolysis in patients with which enzymatic deficiency?

• Thioredoxin reductase deficiency
• Glucose-6-phosphate dehydrogenase (G6PD) deficiency
• Catalase deficiency
• Superoxide dismutase deficiency

Correct Answer: Glucose-6-phosphate dehydrogenase (G6PD) deficiency

Q5. Which isoform of vitamin E is most abundant and biologically active in human tissues?

• Gamma-tocotrienol
• Delta-tocopherol
• Alpha-tocopherol
• Beta-tocotrienol

Correct Answer: Alpha-tocopherol

Q6. Vitamin E protects cell membranes primarily by which mechanism?

• Inhibiting protein oxidation directly
• Acting as a chain‑breaking lipid peroxyl radical scavenger
• Promoting hydroxyl radical formation
• Chelating transition metals in the cytosol

Correct Answer: Acting as a chain‑breaking lipid peroxyl radical scavenger

Q7. Clinical manifestations of severe vitamin E deficiency commonly include:

• Rickets and bone deformities
• Hemolytic anemia and neuromuscular abnormalities (ataxia, peripheral neuropathy)
• Hyperpigmentation and photosensitivity
• Hypocoagulability due to vitamin K excess

Correct Answer: Hemolytic anemia and neuromuscular abnormalities (ataxia, peripheral neuropathy)

Q8. High-dose vitamin E supplementation can increase bleeding risk by interacting with which vitamin pathway?

• Vitamin D activation
• Vitamin K clotting factor gamma‑carboxylation
• Vitamin B12 absorption
• Vitamin A metabolism

Correct Answer: Vitamin K clotting factor gamma‑carboxylation

Q9. The hepatic protein that selectively incorporates alpha‑tocopherol for plasma distribution is called:

• Albumin
• Tocopherol transfer protein (TTP)
• Apolipoprotein E
• Microsomal triglyceride transfer protein (MTP)

Correct Answer: Tocopherol transfer protein (TTP)

Q10. Which cytochrome P450 isoform is primarily implicated in vitamin E omega‑hydroxylation and catabolism?

• CYP3A4
• CYP2D6
• CYP4F2
• CYP1A2

Correct Answer: CYP4F2

Q11. Alpha‑lipoic acid is metabolically reduced in cells to which active dithiol form?

• Oxidized lipoamide
• Dihydrolipoic acid (DHLA)
• Ascorbyl‑lipoate
• Tetrahydrolipoate

Correct Answer: Dihydrolipoic acid (DHLA)

Q12. A distinctive pharmacokinetic property of alpha‑lipoic acid is:

• Strict water solubility only
• Strict lipid solubility only
• Both water and lipid solubility allowing distribution in multiple compartments
• Inability to cross cell membranes

Correct Answer: Both water and lipid solubility allowing distribution in multiple compartments

Q13. Alpha‑lipoic acid has the strongest clinical evidence for use in which condition?

• Acute bacterial infections
• Diabetic peripheral neuropathy
• Osteoarthritis pain
• Hyperthyroidism management

Correct Answer: Diabetic peripheral neuropathy

Q14. Regarding stereochemistry of alpha‑lipoic acid, which statement is correct?

• The S‑enantiomer is the natural, more active form
• The R‑enantiomer is the naturally occurring and more biologically active form
• Only the racemate is biologically active
• Stereochemistry is irrelevant to activity

Correct Answer: The R‑enantiomer is the naturally occurring and more biologically active form

Q15. One antioxidant action unique to dihydrolipoic acid (reduced lipoic acid) is:

• Direct scavenging of hydroxyl radicals only in membranes
• Regeneration of other antioxidants including glutathione, vitamin C and vitamin E
• Inhibition of mitochondrial ATP synthase
• Permanent chelation of essential trace metals preventing utilization

Correct Answer: Regeneration of other antioxidants including glutathione, vitamin C and vitamin E

Q16. A clinically important interaction of alpha‑lipoic acid is potentiation of which drug effect?

• Anticoagulant action of warfarin (causing hypercoagulability)
• Hypoglycemic effect of insulin or oral hypoglycemics
• Bronchodilation caused by beta‑agonists
• Serotonin syndrome with SSRIs

Correct Answer: Hypoglycemic effect of insulin or oral hypoglycemics

Q17. The rate‑limiting enzyme in pineal melatonin synthesis from serotonin is:

• Hydroxyindole‑O‑methyltransferase (HIOMT)
• Monoamine oxidase
• N‑acetyltransferase (arylalkylamine N‑acetyltransferase, AANAT)
• Tyrosine hydroxylase

Correct Answer: N‑acetyltransferase (arylalkylamine N‑acetyltransferase, AANAT)

Q18. Melatonin exerts many of its physiological effects via which receptors?

• MT1 and MT2 G‑protein coupled receptors
• Alpha and beta adrenergic receptors
• NMDA and AMPA receptors
• Estrogen receptor alpha and beta

Correct Answer: MT1 and MT2 G‑protein coupled receptors

Q19. The primary hepatic enzyme responsible for melatonin metabolism is:

• CYP2C9
• CYP1A2
• Monoamine oxidase A
• Flavin monooxygenase (FMO3)

Correct Answer: CYP1A2

Q20. The most evidence‑based clinical use of melatonin in therapeutics is for:

• Management of chronic bacterial infections
• Treatment of circadian rhythm sleep disorders and jet lag
• Long‑term treatment of major depressive disorder as monotherapy
• Primary therapy for hypertension

Correct Answer: Treatment of circadian rhythm sleep disorders and jet lag

Q21. Which antioxidant pair illustrates a physiological regeneration cycle in membranes?

• Vitamin C regenerating vitamin E from the tocopheroxyl radical
• Vitamin E regenerating vitamin C directly without electron transfer
• Alpha‑lipoic acid converting into vitamin K
• Melatonin regenerating glutathione peroxidase

Correct Answer: Vitamin C regenerating vitamin E from the tocopheroxyl radical

Q22. Which compound can help regenerate intracellular glutathione by acting as a redox couple?

• Ascorbyl palmitate
• Dihydrolipoic acid (reduced alpha‑lipoic acid)
• Tocotrienol‑quinone
• Melatonin sulfate

Correct Answer: Dihydrolipoic acid (reduced alpha‑lipoic acid)

Q23. Under certain conditions vitamin C can act as a pro‑oxidant by reducing Fe3+ to Fe2+, promoting which reaction?

• Glutathione synthesis
• Fenton reaction producing hydroxyl radicals
• Enzymatic hydroxylation of steroids
• Beta‑oxidation of fatty acids

Correct Answer: Fenton reaction producing hydroxyl radicals

Q24. Which vitamin C derivative is commonly used in topical skin formulations for improved stability and lipophilicity?

• Sodium ascorbate
• Ascorbyl palmitate (lipophilic ester)
• Dehydroascorbic acid
• Ascorbic acid‑2‑gluconate (unmodified)

Correct Answer: Ascorbyl palmitate (lipophilic ester)

Q25. The most frequent adverse effects reported with high oral vitamin C intake are:

• Severe hypertension and arrhythmias
• Gastrointestinal upset and osmotic diarrhea
• Profound sedation
• Cholestatic jaundice

Correct Answer: Gastrointestinal upset and osmotic diarrhea

Q26. Tocotrienols differ from tocopherols structurally by having:

• A saturated phytyl tail instead of an isoprenoid tail
• An unsaturated isoprenoid side chain (double bonds in the tail)
• A polar phosphate group on the chromanol ring
• No chromanol head group

Correct Answer: An unsaturated isoprenoid side chain (double bonds in the tail)

Q27. Which enzyme reduces monodehydroascorbate back to ascorbate using NAD(P)H?

• Glutathione reductase
• Monodehydroascorbate reductase (MDAR)
• Ascorbate oxidase
• Thioredoxin peroxidase

Correct Answer: Monodehydroascorbate reductase (MDAR)

Q28. Concomitant use of which drug can markedly raise melatonin plasma levels by inhibiting its metabolism?

• Rifampin
• Fluvoxamine (CYP1A2 inhibitor)
• Phenobarbital
• Carbamazepine

Correct Answer: Fluvoxamine (CYP1A2 inhibitor)

Q29. Which antioxidant has been studied at high intravenous doses as an adjunct in some oncology protocols?

• Alpha‑tocopherol oral tablets only
• High‑dose intravenous vitamin C (ascorbate)
• Topical melatonin creams
• Oral alpha‑lipoic acid lozenges

Correct Answer: High‑dose intravenous vitamin C (ascorbate)

Q30. The oral bioavailability of vitamin C is best described as:

• Linear and unlimited with increasing dose
• Saturable due to SVCT transporters, with decreased fractional absorption at high doses
• Completely negligible; only IV administration achieves plasma levels
• Unaffected by dose or intestinal transporters

Correct Answer: Saturable due to SVCT transporters, with decreased fractional absorption at high doses

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