Lactic acid is a key alpha-hydroxy acid widely studied in pharmaceutical sciences. This concise guide on Structure and uses of lactic acid MCQs With Answer covers chemical structure (C3H6O3), stereochemistry (L- and D-forms), physicochemical properties (pKa, solubility), metabolism, and pharmaceutical applications such as pH regulation, exfoliation, humectant action, antimicrobial effects, and the use of lactic acid derivatives like lactate salts and polylactic acid (PLA) in drug delivery. Designed for B. Pharm students, the material balances core theory and formulation practice to build exam-ready understanding and practical insight. Each question includes answer keys to reinforce learning and exam preparation. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. Which of the following is the correct molecular formula of lactic acid?
- C2H4O2
- C3H6O3
- C4H8O4
- CH3COOH
Correct Answer: C3H6O3
Q2. Lactic acid belongs to which functional class of organic compounds?
- Alpha-keto acid
- Beta-hydroxy acid
- Alpha-hydroxy acid
- Gamma-lactone
Correct Answer: Alpha-hydroxy acid
Q3. What is the IUPAC name of lactic acid?
- 2-Hydroxypropanoic acid
- 3-Hydroxybutanoic acid
- 2-Oxopropanoic acid
- Propanoic acid
Correct Answer: 2-Hydroxypropanoic acid
Q4. Which stereochemical forms of lactic acid are biologically relevant?
- Only meso form
- L-(S)- and D-(R)- enantiomers
- Only racemic mixture
- E and Z isomers
Correct Answer: L-(S)- and D-(R)- enantiomers
Q5. The stereocenter in lactic acid is located at which carbon?
- Carboxyl carbon (C1)
- Methyl carbon (C3)
- Alpha carbon bearing hydroxyl (C2)
- Terminal hydrogen-bearing carbon
Correct Answer: Alpha carbon bearing hydroxyl (C2)
Q6. What is the approximate pKa of the carboxyl group of lactic acid?
- ~2.4
- ~3.8
- ~9.5
- ~12.0
Correct Answer: ~3.8
Q7. In pharmaceutical formulations lactic acid is commonly used as:
- Oxidizing agent
- pH regulator and buffering agent
- Strong base
- Anticoagulant
Correct Answer: pH regulator and buffering agent
Q8. Which enzyme catalyzes the interconversion of pyruvate and lactate in human metabolism?
- Alcohol dehydrogenase
- Lactate dehydrogenase
- Pyruvate kinase
- Hexokinase
Correct Answer: Lactate dehydrogenase
Q9. Lactic acid produced industrially at large scale by fermentation typically yields which optical form?
- Pure D-lactic acid only
- Pure L-lactic acid only
- EITHER L-lactic acid (via Lactobacillus species) or racemic mixture depending on process
- Only meso-lactic acid
Correct Answer: EITHER L-lactic acid (via Lactobacillus species) or racemic mixture depending on process
Q10. Which property makes lactic acid useful as an exfoliant in topical formulations?
- Antiviral activity
- Alpha-hydroxy acid increases desquamation of stratum corneum
- Strong antiseptic effect
- High molecular weight polymerization
Correct Answer: Alpha-hydroxy acid increases desquamation of stratum corneum
Q11. The salt form of lactic acid commonly used in formulations is called:
- Lactamide
- Lactate
- Lactic ester
- Lactone
Correct Answer: Lactate
Q12. Which of the following is a major advantage of using polylactic acid (PLA) in drug delivery?
- Non-biodegradable in vivo
- Biodegradable and biocompatible polymer for controlled release
- Highly water-soluble small molecule carrier
- Instant systemic absorption enhancer
Correct Answer: Biodegradable and biocompatible polymer for controlled release
Q13. Lactic acid acts as an antimicrobial agent primarily due to:
- Alkaline pH increase
- Lowering pH and undissociated acid diffusing into microbes
- Generating reactive oxygen species
- Chelation of metal ions
Correct Answer: Lowering pH and undissociated acid diffusing into microbes
Q14. Which reaction type converts lactic acid monomers into polylactic acid (PLA) with high molecular weight?
- Aldol condensation
- Free radical polymerization
- Ring-opening polymerization of lactide
- Radical-initiated crosslinking
Correct Answer: Ring-opening polymerization of lactide
Q15. Lactic acid is classified in food industry with which E-number?
- E200
- E270
- E330
- E500
Correct Answer: E270
Q16. Which analytical technique is most appropriate to determine enantiomeric purity of lactic acid?
- Normal GC on achiral column
- Optical rotation measurement or chiral HPLC
- IR spectroscopy only
- Elemental analysis
Correct Answer: Optical rotation measurement or chiral HPLC
Q17. In buffer preparation, a suitable lactic acid/lactate buffer typically works best around which pH?
- pH 1–2
- pH 3–4
- pH 7–8
- pH 10–11
Correct Answer: pH 3–4
Q18. Lactic acid’s role as a humectant in topical formulations is primarily due to:
- Its high molecular weight
- Ability to attract and retain moisture through hydroxyl groups
- Strong surfactant properties
- Generating foam
Correct Answer: Ability to attract and retain moisture through hydroxyl groups
Q19. Chemical synthesis of lactic acid by hydration of acetaldehyde typically yields:
- Enantiomerically pure L-form
- Enantiomerically pure D-form
- Racemic mixture of D- and L-forms
- Only polymeric PLA
Correct Answer: Racemic mixture of D- and L-forms
Q20. Which group on lactic acid provides the molecule with both hydrophilic and weak acidic properties?
- Methyl group
- Hydroxyl and carboxyl groups
- Aromatic ring
- Amide linkage
Correct Answer: Hydroxyl and carboxyl groups
Q21. Which lactate stereoisomer predominates in human serum under normal physiology?
- D-lactate
- L-lactate
- Equal D and L
- Acetate predominates, not lactate
Correct Answer: L-lactate
Q22. D-lactic acidosis is most likely to cause clinical issues in which situation?
- After ingestion of racemic lactic acid in cosmetics
- In short bowel syndrome with bacterial overgrowth producing D-lactate
- In normal athletic exercise
- After topical application of lactic acid
Correct Answer: In short bowel syndrome with bacterial overgrowth producing D-lactate
Q23. Which of the following best describes lactic acid’s solubility in water?
- Insoluble
- Practically insoluble
- Miscible/very soluble
- Only soluble in organic solvents
Correct Answer: Miscible/very soluble
Q24. The optical rotation of pure L-lactic acid is described as:
- Dextrarotatory (+)
- Levorotatory (−)
- Zero (optically inactive)
- Variable depending on concentration
Correct Answer: Levorotatory (−)
Q25. Lactic acid is categorized as which kind of metabolite during anaerobic glycolysis?
- End product of anaerobic glycolysis from pyruvate
- Primary reactant in beta-oxidation
- Coenzyme in TCA cycle
- Storage polysaccharide precursor
Correct Answer: End product of anaerobic glycolysis from pyruvate
Q26. Which pharmaceutical formulation type commonly uses lactic acid as a penetration enhancer for topical drugs?
- Enteric-coated tablets
- Transdermal gels and creams
- Parenteral injectables
- Dry powder inhalers
Correct Answer: Transdermal gels and creams
Q27. During manufacturing, removal of moisture from lactic acid can cause which undesirable reaction?
- Hydrolysis to formic acid
- Polymerization to form oligomers and esters
- Oxidation to pyruvate
- Isomerization to acetic acid
Correct Answer: Polymerization to form oligomers and esters
Q28. In stability studies, which factor most accelerates degradation of lactic acid solutions?
- Low temperature
- Exposure to strong acids or bases and high temperature
- Storage in amber glass
- Using inert gas headspace
Correct Answer: Exposure to strong acids or bases and high temperature
Q29. Which natural source is commonly used for fermentation-based production of L-lactic acid?
- Petroleum feedstocks only
- Lactobacillus species fermenting carbohydrates
- Direct extraction from animal blood
- Chemical oxidation of ethanol
Correct Answer: Lactobacillus species fermenting carbohydrates
Q30. Lactic acid’s role as a chelating agent is:
- Strong chelator like EDTA
- Moderate chelating ability due to hydroxyl and carboxyl groups
- None; it cannot chelate metals
- Only chelates heavy metals irreversibly
Correct Answer: Moderate chelating ability due to hydroxyl and carboxyl groups
Q31. Which of the following is a common topical concentration range for lactic acid as an AHA in cosmetics/dermatology?
- 0.01–0.1%
- 2–10%
- 50–70%
- 90–100%
Correct Answer: 2–10%
Q32. Which analytical method is best suited to quantify lactic acid in biological fluids?
- Turbidity measurement
- Enzymatic assay using lactate oxidase or lactate dehydrogenase
- pH paper test only
- Melting point determination
Correct Answer: Enzymatic assay using lactate oxidase or lactate dehydrogenase
Q33. In polymeric drug delivery, PLA degrades primarily by which mechanism?
- Enzymatic proteolysis
- Hydrolytic cleavage of ester bonds
- Photodegradation only
- Ion-exchange resins
Correct Answer: Hydrolytic cleavage of ester bonds
Q34. Which type of bond formation links lactic acid monomers in polylactic acid?
- Peptide bonds
- Ester bonds
- Ether bonds
- Amide bonds
Correct Answer: Ester bonds
Q35. Which lactic acid derivative is commonly used as an excipient to adjust tonicity or pH in parenteral solutions?
- Polyethylene glycol
- Sodium lactate
- Lactic acid polymer (PLA)
- Lactulose
Correct Answer: Sodium lactate
Q36. Lactic acid’s mechanism as an AHA influencing skin includes stimulation of:
- Melanin production exclusively
- Keratinocyte proliferation and exfoliation, and stimulation of collagen synthesis
- Only sebaceous gland activity
- Complete epidermal necrosis
Correct Answer: Keratinocyte proliferation and exfoliation, and stimulation of collagen synthesis
Q37. Which safety concern is most relevant for topical lactic acid at high concentrations?
- Systemic toxicity due to absorption
- Skin irritation and chemical burns
- Permanent staining of clothes
- Respiratory depression
Correct Answer: Skin irritation and chemical burns
Q38. Lactic acid participates in the Cori cycle by being converted to:
- Fatty acids in adipose tissue
- Pyruvate in the liver and then to glucose via gluconeogenesis
- Alanine directly in muscles
- Acetone in the liver
Correct Answer: Pyruvate in the liver and then to glucose via gluconeogenesis
Q39. Which of the following is true about lactide, the cyclic dimer used for PLA production?
- It is a monomer of acrylic acid
- It is formed by condensation of two lactic acid units and can undergo ring-opening polymerization
- It is non-polymerizable
- It contains an amide linkage
Correct Answer: It is formed by condensation of two lactic acid units and can undergo ring-opening polymerization
Q40. In drug-excipient compatibility, lactic acid may react with which class of excipients to form esters?
- Primary alcohol-containing excipients
- Inert glass containers only
- Metallic fillers
- Nonpolar hydrocarbons
Correct Answer: Primary alcohol-containing excipients
Q41. Which physiological condition is associated with elevated blood lactate levels?
- Respiratory alkalosis
- Lactic acidosis due to hypoxia or sepsis
- Hypoglycemia-free state
- Hypermagnesemia
Correct Answer: Lactic acidosis due to hypoxia or sepsis
Q42. Which of the following is an advantage of using L-lactic acid over racemic lactic acid in biomedical implants?
- Faster uncontrolled degradation
- Improved biocompatibility and predictable degradation due to stereoregularity
- Lower melting point that is clinically irrelevant
- Higher toxicity
Correct Answer: Improved biocompatibility and predictable degradation due to stereoregularity
Q43. A major metabolic fate of lactate in well-oxygenated muscle is:
- Converted to ethanol
- Oxidized to pyruvate and entering TCA cycle for ATP production
- Excreted unchanged in urine
- Converted to glycogen directly
Correct Answer: Oxidized to pyruvate and entering TCA cycle for ATP production
Q44. Which solvent would be least suitable to dissolve lactic acid for analysis in typical aqueous assays?
- Water
- Methanol
- Hexane
- Ethanol
Correct Answer: Hexane
Q45. For sterile parenteral use, sodium lactate serves chiefly as:
- An osmotic diuretic
- A buffer and alkalinizing agent to correct acidosis
- An anesthetic
- A surfactant
Correct Answer: A buffer and alkalinizing agent to correct acidosis
Q46. Lactic acid’s antimicrobial efficacy is enhanced when paired with which factor?
- Increased formulation pH
- Lower formulation pH and presence of undissociated acid
- Higher water activity only
- Decreased contact time
Correct Answer: Lower formulation pH and presence of undissociated acid
Q47. In formulation chemistry, esterification of lactic acid with alcohols yields which useful class of compounds?
- Polyalcohols
- Lactate esters used as solvents or plasticizers
- Peptide analogs
- Strong oxidants
Correct Answer: Lactate esters used as solvents or plasticizers
Q48. Which quality control parameter is most important to monitor for lactic acid raw material used in PLA synthesis?
- Optical purity and water content
- Color only
- Presence of heavy metals exclusively
- Particle size distribution
Correct Answer: Optical purity and water content
Q49. Lactic acid’s compatibility with metal implants is often limited due to:
- Its inertness toward metals
- Potential to corrode metals by lowering local pH
- Its ability to form protective metal-lactate films preventing corrosion
- Its volatility
Correct Answer: Potential to corrode metals by lowering local pH
Q50. Which statement best summarizes why lactic acid and its derivatives are important in pharmaceutical sciences?
- They are obsolete and rarely used
- They combine versatile chemistry (AHA, buffering, esterification) with biocompatibility and biodegradability for formulations and delivery systems
- They are only useful as strong oxidizing preservatives
- They are purely cosmetic agents without therapeutic relevance
Correct Answer: They combine versatile chemistry (AHA, buffering, esterification) with biocompatibility and biodegradability for formulations and delivery systems
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