Understanding the Classification of disinfectants MCQs With Answer is essential for B.Pharm students preparing for exams and hospital pharmacy practice. This concise, SEO-friendly introduction covers disinfectant classification, modes of action, spectrum (bactericidal, virucidal, fungicidal, sporicidal), examples like halogens, phenolics, quaternary ammonium compounds, aldehydes and oxidizing agents, and practical considerations such as concentration, contact time, organic load and instrument compatibility. These MCQs emphasize mechanisms, safety, environmental impact and selection criteria relevant to sterilization and infection control. The questions are tailored to strengthen theoretical knowledge and clinical application for pharmacy students. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. Which basis is commonly used to classify disinfectants when grouping them into phenolics, halogens, oxidizing agents and aldehydes?
- Chemical structure and mechanism of action
- Manufacturer brand name
- Color and odor profile
- Cost and availability
Correct Answer: Chemical structure and mechanism of action
Q2. Which disinfectant class primarily exerts action by protein denaturation and membrane disruption and includes ethanol and isopropanol?
- Halogens
- Alcohols
- Alkylating agents
- Phenolics
Correct Answer: Alcohols
Q3. Which agent is considered a high-level disinfectant capable of inactivating mycobacteria, viruses, fungi and some spores with appropriate exposure?
- Quaternary ammonium compounds
- Glutaraldehyde
- Sodium hypochlorite at household concentration
- Gentle soap solutions
Correct Answer: Glutaraldehyde
Q4. Phenolic disinfectants are best described by which primary mechanism?
- Alkylation of nucleic acids
- Oxidation of sulfhydryl groups
- Disruption of cell membranes and denaturation of proteins
- Chelation of divalent cations
Correct Answer: Disruption of cell membranes and denaturation of proteins
Q5. Which halogen disinfectant is widely used as a hospital surface disinfectant and acts through oxidation and halogenation?
- Sodium hypochlorite
- Hydrogen peroxide
- Glutaraldehyde
- Chlorhexidine
Correct Answer: Sodium hypochlorite
Q6. Quaternary ammonium compounds are least effective against which organism type without supporting agents?
- Gram-positive bacteria
- Enveloped viruses
- Non-enveloped viruses and mycobacteria
- Fungi
Correct Answer: Non-enveloped viruses and mycobacteria
Q7. Which disinfectant class is known for cross-linking proteins and nucleic acids, often used for high-level disinfection but is toxic and requires ventilation?
- Aldehydes (e.g., glutaraldehyde, formaldehyde)
- Alcohols
- Biguanides (e.g., chlorhexidine)
- Quats
Correct Answer: Aldehydes (e.g., glutaraldehyde, formaldehyde)
Q8. Peracetic acid is favored in some settings because of which advantage over glutaraldehyde?
- Lower microbicidal activity
- Faster biodegradation to non-toxic residues
- Less effective against spores
- Longer residual antimicrobial action
Correct Answer: Faster biodegradation to non-toxic residues
Q9. The phenol coefficient is a comparative measure of a disinfectant’s activity relative to which reference compound?
- Ethanol
- Phenol (carbolic acid)
- Sodium hypochlorite
- Hydrogen peroxide
Correct Answer: Phenol (carbolic acid)
Q10. Which factor most commonly reduces the efficacy of oxidizing disinfectants like hypochlorite and peroxygen compounds?
- Low temperature
- Presence of organic matter and blood
- High pH
- Lack of light
Correct Answer: Presence of organic matter and blood
Q11. What is the primary antimicrobial action of hydrogen peroxide and peracetic acid?
- Membrane solubilization
- Protein coagulation by alcoholysis
- Production of free radicals leading to oxidation of cellular components
- DNA intercalation
Correct Answer: Production of free radicals leading to oxidation of cellular components
Q12. Which disinfectant is commonly described as surfactant-based and is inactivated by anionic detergents?
- Quaternary ammonium compounds
- Aldehydes
- Chlorine dioxide
- Hydrogen peroxide
Correct Answer: Quaternary ammonium compounds
Q13. Which disinfectant is recommended for effective inactivation of hepatitis B and HIV on environmental surfaces with proper concentration and contact time?
- Alcohol 70% applied briefly
- Dilute sodium hypochlorite (bleach) solutions
- Plain water rinse
- Soap and towel drying
Correct Answer: Dilute sodium hypochlorite (bleach) solutions
Q14. Which parameter is most critical when choosing a disinfectant for heat-sensitive medical instruments?
- Color of the disinfectant
- Instrument manufacturer’s material compatibility and the disinfectant’s level (high/intermediate/low)
- Cost per liter only
- Smell intensity
Correct Answer: Instrument manufacturer’s material compatibility and the disinfectant’s level (high/intermediate/low)
Q15. Biguanides such as chlorhexidine primarily act by which mechanism?
- Denaturing nucleic acids
- Disrupting cell membrane integrity and causing leakage of intracellular components
- Chelation of metal ions
- Oxidative free radical formation
Correct Answer: Disrupting cell membrane integrity and causing leakage of intracellular components
Q16. Which disinfectant is considered sporicidal at appropriate concentration and contact time and is used to sterilize endoscopes when combined with appropriate protocols?
- 70% ethanol
- Peracetic acid
- Quaternary ammonium compounds
- Soap solution
Correct Answer: Peracetic acid
Q17. What is the effect of increasing temperature on the activity of most chemical disinfectants?
- Generally increases microbicidal activity and shortens required contact time
- Always inactivates the disinfectant
- No effect whatsoever
- Causes precipitate formation making them ineffective
Correct Answer: Generally increases microbicidal activity and shortens required contact time
Q18. Which disinfectant is often contraindicated for use on porous surfaces because it is corrosive and unstable in the presence of organic matter?
- Sodium hypochlorite (bleach)
- Chlorhexidine
- Isopropyl alcohol 70%
- Quaternary ammonium compounds
Correct Answer: Sodium hypochlorite (bleach)
Q19. The term ‘intermediate-level disinfectant’ refers to agents that reliably inactivate which of the following?
- All bacterial spores
- Mycobacteria, most viruses and fungi but not necessarily spores
- Only vegetative Gram-positive bacteria
- Only protozoa
Correct Answer: Mycobacteria, most viruses and fungi but not necessarily spores
Q20. Which disinfectant class is most associated with residual activity on skin and surfaces due to adsorption and slow release?
- Alcohols
- Phenolics
- Quaternary ammonium compounds
- Peroxygens
Correct Answer: Quaternary ammonium compounds
Q21. Which chemical is commonly used as an intermediate- to high-level disinfectant for heat-sensitive instruments and requires activation and careful handling due to toxicity?
- Glutaraldehyde
- 70% ethanol
- Soap
- Benzalkonium chloride
Correct Answer: Glutaraldehyde
Q22. In the presence of organic load, which approach improves disinfection efficacy of hypochlorite solutions?
- Using lower concentration
- Pre-cleaning to remove organic material followed by disinfectant application
- Adding soap before hypochlorite
- Reducing contact time
Correct Answer: Pre-cleaning to remove organic material followed by disinfectant application
Q23. Which disinfectant is known to be inactivated by hard water and requires use with soft or deionized water for optimal activity?
- Chlorine dioxide
- Quaternary ammonium compounds
- Phenolics
- Hydrogen peroxide
Correct Answer: Quaternary ammonium compounds
Q24. Which statement best differentiates sterilization from disinfection?
- Sterilization reduces microbial load; disinfection achieves complete absence of viable microbes
- Sterilization kills all forms of microbial life including spores; disinfection eliminates many or all pathogenic microorganisms except necessarily all spores
- Disinfection uses heat only; sterilization uses chemicals only
- They are synonymous terms
Correct Answer: Sterilization kills all forms of microbial life including spores; disinfection eliminates many or all pathogenic microorganisms except necessarily all spores
Q25. Which disinfectant agent is effective as a tuberculocidal and is commonly used for environmental disinfection at appropriate concentrations?
- Phenol at low dilution
- Chlorine-based compounds (sodium hypochlorite)
- Soap and water
- Lower-alcohol rinses
Correct Answer: Chlorine-based compounds (sodium hypochlorite)
Q26. What is the main limitation of using alcohols as sole disinfectants for contaminated instruments?
- They are sporicidal
- They evaporate quickly and have no residual activity; they are not reliably sporicidal
- They form stable residues that damage tissue
- They are ineffective against vegetative bacteria
Correct Answer: They evaporate quickly and have no residual activity; they are not reliably sporicidal
Q27. Which disinfectant is commonly used in water treatment and acts by inactivating enzymes and compromising membrane integrity via halogenation?
- Iodophors
- Chlorine/chloramines
- Chlorhexidine
- Benzalkonium chloride
Correct Answer: Chlorine/chloramines
Q28. Which of the following is an advantage of iodophors over free iodine solution?
- Greater corrosiveness
- Lower stability in presence of organic matter
- Less staining, improved solubility and prolonged release of iodine
- Reduced antimicrobial spectrum
Correct Answer: Less staining, improved solubility and prolonged release of iodine
Q29. Which disinfectant requires a neutralizer before microbiological sampling to avoid carryover effects?
- Chlorhexidine
- Sodium hypochlorite
- Alcohol
- Soap
Correct Answer: Sodium hypochlorite
Q30. Which chemical disinfectant is commonly used for cold sterilization of heat-sensitive instruments and requires at least 10–12 hours immersion to achieve sterilization?
- 2% glutaraldehyde (when used as sterilant)
- 70% isopropyl alcohol
- Quaternary ammonium compounds
- Phenolic detergents
Correct Answer: 2% glutaraldehyde (when used as sterilant)
Q31. Which class of disinfectant is most likely to persist as a toxic residue on medical devices requiring thorough rinsing after exposure?
- Peracetic acid
- Glutaraldehyde and formaldehyde
- Alcohol
- Soap
Correct Answer: Glutaraldehyde and formaldehyde
Q32. Biofilms on surfaces reduce disinfectant efficacy primarily because:
- Biofilms increase disinfectant solubility
- Cells within biofilms are protected by extracellular polymeric substances that impede penetration
- Biofilms convert disinfectants to more potent agents
- Disinfectants selectively kill biofilm-forming cells only
Correct Answer: Cells within biofilms are protected by extracellular polymeric substances that impede penetration
Q33. Which disinfectant would be least appropriate for use on flexible endoscopes due to material incompatibility and toxicity?
- Peracetic acid with validated process
- 2% glutaraldehyde with appropriate rinsing
- Concentrated sodium hypochlorite bleach
- Approved enzymatic detergent followed by high-level disinfectant
Correct Answer: Concentrated sodium hypochlorite bleach
Q34. Which statement about aldehydes like glutaraldehyde is true?
- They are non-toxic and safe for prolonged inhalation exposure
- They cross-link proteins and nucleic acids, providing broad antimicrobial activity but are irritant and potentially sensitizing
- They are rapidly biodegradable and leave no residues
- They are ineffective against mycobacteria
Correct Answer: They cross-link proteins and nucleic acids, providing broad antimicrobial activity but are irritant and potentially sensitizing
Q35. Which disinfectant is most suitable for rapid skin antisepsis prior to injections?
- Hydrogen peroxide 3% left to evaporate for seconds
- 70% isopropyl alcohol applied and allowed to dry
- Sodium hypochlorite on intact skin
- 2% glutaraldehyde applied directly to skin
Correct Answer: 70% isopropyl alcohol applied and allowed to dry
Q36. Which chemical group of disinfectants acts by alkylation of protein and nucleic acid functional groups and is often used for instrument sterilization in gas form?
- Alcohols
- Alkylating agents such as ethylene oxide (gas sterilant)
- Quaternary ammonium compounds
- Phenolics
Correct Answer: Alkylating agents such as ethylene oxide (gas sterilant)
Q37. What is the recommended practice when using chemical disinfectants to ensure consistent efficacy?
- Ignore manufacturer label for concentration and contact time
- Follow validated manufacturer instructions for concentration, contact time, and preparation; perform pre-cleaning and monitor using indicators when applicable
- Always dilute to half the recommended concentration to save costs
- Mix different disinfectants to broaden spectrum without compatibility testing
Correct Answer: Follow validated manufacturer instructions for concentration, contact time, and preparation; perform pre-cleaning and monitor using indicators when applicable
Q38. Which disinfectant class typically has residual activity on skin and can be used for hand scrubs with sustained effect?
- Alcohols alone
- Biguanides (e.g., chlorhexidine) often combined with alcohol for residual effect
- Hypochlorite solutions
- Peracetic acid
Correct Answer: Biguanides (e.g., chlorhexidine) often combined with alcohol for residual effect
Q39. The presence of bio-burden (high microbial load) on an object prior to disinfection generally necessitates which step?
- Immediate application of disinfectant without cleaning
- Pre-cleaning and mechanical removal of debris followed by appropriate disinfection
- Only drying the object in air
- Applying lower disinfectant concentration
Correct Answer: Pre-cleaning and mechanical removal of debris followed by appropriate disinfection
Q40. Which disinfectant is preferred for decontamination of surfaces contaminated with Clostridioides difficile spores in healthcare settings?
- 70% ethanol
- Standard quaternary ammonium disinfectants
- Sodium hypochlorite at appropriate concentration or sporicidal agents such as peracetic acid
- Chlorhexidine gluconate wipes
Correct Answer: Sodium hypochlorite at appropriate concentration or sporicidal agents such as peracetic acid
Q41. Which disinfectant parameter is most relevant when evaluating environmental toxicity and disposal concerns?
- pH only
- Biodegradability, formation of toxic by-products (e.g., chlorinated organics) and regulatory disposal requirements
- Odor intensity
- Color of the solution
Correct Answer: Biodegradability, formation of toxic by-products (e.g., chlorinated organics) and regulatory disposal requirements
Q42. Which test organism is commonly used to evaluate tuberculocidal activity of disinfectants?
- Staphylococcus aureus
- Mycobacterium tuberculosis or surrogate mycobacteria such as Mycobacterium bovis BCG
- Escherichia coli
- Candida albicans
Correct Answer: Mycobacterium tuberculosis or surrogate mycobacteria such as Mycobacterium bovis BCG
Q43. Which statement about chlorhexidine is correct?
- It is a potent sporicide at room temperature
- It has persistent antiseptic properties on skin but limited activity against spores and non-enveloped viruses
- It rapidly oxidizes to form toxic gases
- It is the preferred agent for decontaminating blood spills
Correct Answer: It has persistent antiseptic properties on skin but limited activity against spores and non-enveloped viruses
Q44. Which disinfectant is best for use in sterilizing heat-sensitive powders and oils where liquid immersion is impractical?
- Autoclave
- Ethylene oxide gas sterilization
- 70% ethanol bath
- Quaternary ammonium wipe
Correct Answer: Ethylene oxide gas sterilization
Q45. Which disinfectant is inactivated by catalase-producing organisms more rapidly due to enzymatic degradation?
- Hydrogen peroxide
- Glutaraldehyde
- Phenol
- Alcohol
Correct Answer: Hydrogen peroxide
Q46. Which disinfectant class commonly used for environmental disinfection releases a toxic gas if mixed with acids and therefore requires caution?
- Quaternary ammonium compounds
- Chlorine-releasing agents such as sodium hypochlorite
- Alcohols
- Peracetic acid
Correct Answer: Chlorine-releasing agents such as sodium hypochlorite
Q47. What is a major occupational hazard associated with prolonged exposure to formaldehyde used as a disinfectant or sterilant?
- Hypersalivation
- Respiratory irritation, sensitization and potential carcinogenicity
- Improved lung function
- Enhanced cognitive performance
Correct Answer: Respiratory irritation, sensitization and potential carcinogenicity
Q48. Which parameter must be validated when a hospital implements a new high-level disinfectant process for flexible endoscopes?
- Color of the disinfectant solution only
- Manufacturer training, contact time, concentration, material compatibility, rinsing procedures and biological indicators or periodic efficacy checks
- How pleasant the scent is
- Whether it evaporates quickly
Correct Answer: Manufacturer training, contact time, concentration, material compatibility, rinsing procedures and biological indicators or periodic efficacy checks
Q49. Which disinfectant type provides rapid broad-spectrum antimicrobial action but lacks residual activity on surfaces and is unsuitable for prolonged protection?
- Alcohols
- Quaternary ammonium compounds with polymer modifiers
- Iodophors with sustained release
- Chlorinated compounds with persistent residues
Correct Answer: Alcohols
Q50. Which concept describes the concentration and exposure time relationship necessary to achieve a given log reduction of microorganisms?
- Phenol coefficient
- Concentration x Time (Ct) or concentration–time relationship
- Minimum inhibitory color
- Environmental persistence index
Correct Answer: Concentration x Time (Ct) or concentration–time relationship
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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