Physical methods of sterilization MCQs With Answer: A concise, exam-focused introduction for B.Pharm students. Physical sterilization methods—moist heat (autoclave), dry heat (hot air oven), filtration, ultraviolet and ionizing radiation—are core topics in pharmaceutical microbiology and sterile production. This introduction emphasizes mechanisms of action, critical parameters (temperature, time, pressure, pore size, radiation dose), validation tools (biological indicators, Bowie-Dick test) and selection criteria for heat-labile versus heat-stable materials. Targeted MCQs reinforce understanding of sterility assurance levels, filter selection, depyrogenation and practical applications in aseptic processing. Strengthen your conceptual knowledge and exam readiness with focused practice. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the primary mechanism by which moist heat (autoclave) sterilizes microorganisms?
- Oxidation of cellular components
- Coagulation and denaturation of proteins
- Physical removal through filtration
- DNA strand breakage by ionizing radiation
Correct Answer: Coagulation and denaturation of proteins
Q2. Standard autoclave parameters commonly used for sterilization are:
- 121°C for 15 minutes at 15 psi
- 160°C for 2 hours at atmospheric pressure
- 254 nm UV exposure for 30 minutes
- 25 kGy gamma radiation dose
Correct Answer: 121°C for 15 minutes at 15 psi
Q3. Which organism is typically used as a biological indicator for steam autoclave validation?
- Escherichia coli
- Geobacillus stearothermophilus
- Bacillus subtilis var. niger
- Staphylococcus aureus
Correct Answer: Geobacillus stearothermophilus
Q4. Dry heat sterilization in a hot air oven primarily kills microorganisms by:
- Coagulating proteins through moisture
- Oxidation and dry heat denaturation
- Physical filtration of microbes
- Generating thymine dimers in DNA
Correct Answer: Oxidation and dry heat denaturation
Q5. Which of the following is the correct typical condition for dry heat depyrogenation of glassware?
- 121°C for 15 minutes
- 160°C for 2 hours
- 250°C for 30 minutes
- 60°C for 6 hours
Correct Answer: 250°C for 30 minutes
Q6. Which pore size is generally considered the standard for sterilizing-grade membrane filters used for bacteria removal from solutions?
- 0.45 μm
- 0.22 μm
- 0.8 μm
- 5 μm
Correct Answer: 0.22 μm
Q7. Filtration is the preferred physical method for sterilizing which type of pharmaceutical material?
- Non-porous metal surgical instruments
- Heat-labile protein solutions
- Glass ampoules intended for dry heat
- Bulk powders requiring oxidative sterilization
Correct Answer: Heat-labile protein solutions
Q8. HEPA filters are designed to remove particles of which approximate size with high efficiency?
- 0.3 μm
- 10 μm
- 5 nm
- 50 μm
Correct Answer: 0.3 μm
Q9. Which statement correctly distinguishes membrane filters from depth filters?
- Membrane filters have a tortuous matrix for particle entrapment; depth filters are uniform pore membranes
- Membrane filters are typically used for high-load clarification; depth filters provide final sterilization
- Membrane filters have defined pore sizes and are used for sterilizing-grade filtration; depth filters trap particles within a thick matrix
- Membrane filters are made only from glass fibers while depth filters are polymeric
Correct Answer: Membrane filters have defined pore sizes and are used for sterilizing-grade filtration; depth filters trap particles within a thick matrix
Q10. Ultraviolet (UV-C) radiation in sterilization primarily acts by:
- Causing oxidative damage through free radicals
- Producing thymine dimers in DNA that block replication
- Heating cellular components to lethal temperatures
- Physically removing cells via airflow
Correct Answer: Producing thymine dimers in DNA that block replication
Q11. A major limitation of UV irradiation for sterilization is:
- Excessive penetration through dense materials
- Inability to inactivate bacteria
- Poor penetration and shadowing effect on surfaces
- Creation of heat that damages equipment
Correct Answer: Poor penetration and shadowing effect on surfaces
Q12. Ionizing radiation (gamma or electron beam) is commonly used for sterilizing:
- Heat-sensitive single-use medical disposables and certain pharmaceuticals
- Large volumes of aqueous protein solutions requiring filtration
- Surface disinfection where UV is preferred
- Glassware requiring depyrogenation
Correct Answer: Heat-sensitive single-use medical disposables and certain pharmaceuticals
Q13. Typical sterilizing dose for gamma radiation applied to medical devices is approximately:
- 2–5 Gray
- 25 kGy
- 121 kGy
- 0.3 μm
Correct Answer: 25 kGy
Q14. The Bowie-Dick test is primarily used to detect which issue in a prevacuum autoclave?
- Insufficient steam temperature
- Air entrapment and inadequate steam penetration
- Incorrect biological indicator species
- Overexposure to radiation
Correct Answer: Air entrapment and inadequate steam penetration
Q15. Which biological indicator organism is commonly used for dry heat sterilization validation?
- Geobacillus stearothermophilus
- Bacillus atrophaeus (formerly B. subtilis var. niger)
- Escherichia coli
- Pseudomonas aeruginosa
Correct Answer: Bacillus atrophaeus (formerly B. subtilis var. niger)
Q16. The ‘D-value’ in sterilization kinetics refers to:
- The time required to achieve 100% kill
- The decimal reduction time to reduce microbial population by 90%
- The temperature required for sterilization
- The pressure needed to reach boiling point
Correct Answer: The decimal reduction time to reduce microbial population by 90%
Q17. The ‘Z-value’ in thermal inactivation represents:
- The change in time required to reduce microbial count by one log at constant temperature
- The temperature change needed to change the D-value by a factor of ten
- The pressure change that alters sterilization efficacy
- The pore size that retains viruses
Correct Answer: The temperature change needed to change the D-value by a factor of ten
Q18. Sterility Assurance Level (SAL) commonly required for terminally sterilized pharmaceutical products is:
- 10^0
- 10^-3
- 10^-6
- 100%
Correct Answer: 10^-6
Q19. Which test is used to check the integrity of sterilizing membrane filters after use?
- Bacterial endotoxin test
- Bubble point test
- Bowie-Dick test
- pH test
Correct Answer: Bubble point test
Q20. Which of these is NOT a physical method of sterilization?
- Autoclaving
- Gamma irradiation
- Ethylene oxide gas
- Filtration
Correct Answer: Ethylene oxide gas
Q21. What is the main advantage of membrane sterilizing filters over heat sterilization for parenteral solutions?
- They remove endotoxins better than heat
- They allow sterilization of heat-sensitive formulations without thermal degradation
- They are less costly for large volumes of aqueous products
- They add antimicrobial chemicals to the solution
Correct Answer: They allow sterilization of heat-sensitive formulations without thermal degradation
Q22. Which parameter is most critical in ensuring effective steam sterilization?
- Relative humidity of the room
- Contact time, temperature, and steam quality
- Ambient light intensity
- Pore size of packaging material
Correct Answer: Contact time, temperature, and steam quality
Q23. Which of the following correctly describes flash sterilization?
- Short-cycle steam sterilization of unwrapped instruments at higher temperature for a short time
- Low-temperature gas plasma sterilization for delicate devices
- Radiation sterilization using low-dose UV
- Extended dry heat sterilization for glassware
Correct Answer: Short-cycle steam sterilization of unwrapped instruments at higher temperature for a short time
Q24. Which physical method is most effective for pyrogen (endotoxin) removal from glassware?
- Filtration through 0.22 μm membrane
- Dry heat depyrogenation at high temperature
- UV irradiation for 10 minutes
- Standard autoclaving at 121°C for 15 minutes
Correct Answer: Dry heat depyrogenation at high temperature
Q25. For sterile filtration of organic solvents, which type of membrane is typically selected?
- Hydrophilic membrane
- Hydrophobic membrane
- Cellulose acetate for all solvents
- Glass fiber membrane only
Correct Answer: Hydrophobic membrane
Q26. Which of the following best explains why moist heat sterilization is generally more effective than dry heat at the same temperature?
- Steam is heavier than air and compresses microorganisms
- Moist heat transfers energy more efficiently and causes rapid coagulation of proteins
- Dry heat creates oxidative radicals that are less effective
- Moist heat filters out bacteria mechanically
Correct Answer: Moist heat transfers energy more efficiently and causes rapid coagulation of proteins
Q27. What is the primary advantage of using a pre-vacuum (prevac) autoclave cycle over a gravity-displacement cycle?
- Lower energy consumption
- Improved steam penetration and shorter cycle times for wrapped loads
- Ability to sterilize dry powders without moisture
- Better performance for glass depyrogenation
Correct Answer: Improved steam penetration and shorter cycle times for wrapped loads
Q28. Which physical sterilization method is most appropriate for sterilizing antibiotics that are heat-sensitive?
- Dry heat at 160°C
- Autoclaving at 121°C
- Sterile membrane filtration
- Depyrogenation at 250°C
Correct Answer: Sterile membrane filtration
Q29. Which physical method is commonly used to sterilize bulk powders in the pharmaceutical industry?
- Autoclave immersion
- Gamma irradiation
- Membrane filtration
- UV surface exposure
Correct Answer: Gamma irradiation
Q30. Which statement about UV-C (254 nm) germicidal lamps is true?
- They are effective for sterilizing opaque liquids
- They are best suited for surface and air disinfection due to limited penetration
- They are the standard for terminal sterilization of parenterals
- They generate ionizing radiation that penetrates packaging
Correct Answer: They are best suited for surface and air disinfection due to limited penetration
Q31. When choosing a sterilizing filter for a parenteral, which attribute is most critical besides pore size?
- Color of the filter housing
- Filter compatibility with formulation (chemical compatibility and adsorption characteristics)
- Brand popularity
- Electrical conductivity
Correct Answer: Filter compatibility with formulation (chemical compatibility and adsorption characteristics)
Q32. Which physical test provides direct evidence of sterilization rather than just exposure to conditions?
- Chemical indicator tape
- Biological indicator incubation showing kill
- Temperature recorder strip
- Cycle printout
Correct Answer: Biological indicator incubation showing kill
Q33. Which of these correctly describes the relationship between autoclave temperature and exposure time for equivalent sterilization?
- Higher temperature requires longer time for equivalent kill
- Lower temperature always provides better sterilization
- Higher temperature usually requires shorter time for equivalent microbial kill
- Temperature has no effect if pressure is constant
Correct Answer: Higher temperature usually requires shorter time for equivalent microbial kill
Q34. What is the main purpose of placing biological indicators in the most challenging location of a sterilizer load?
- To ensure the indicator is exposed to minimal sterilizing conditions
- To save costs by using fewer indicators
- To verify that the entire load, including hardest-to-sterilize areas, reaches sterilization conditions
- To calibrate the autoclave temperature sensor
Correct Answer: To verify that the entire load, including hardest-to-sterilize areas, reaches sterilization conditions
Q35. Which of the following is true about filter sterilization and endotoxins?
- Standard 0.22 μm sterilizing filters reliably remove endotoxins
- Endotoxins are heat-labile and removed by autoclaving
- Filtration does not remove endotoxins; depyrogenation methods are required
- Endotoxins are removed by UV radiation during filtration
Correct Answer: Filtration does not remove endotoxins; depyrogenation methods are required
Q36. Which indicator only shows a physical change (exposure) but does not confirm sterility?
- Biological indicator
- Chemical indicator tape
- Microbial culture of the load
- Sterility test by membrane filtration
Correct Answer: Chemical indicator tape
Q37. Which of the following best describes ‘sterile filtration’ as applied in aseptic processing?
- Passing solution through a sterilizing-grade membrane to remove microorganisms
- Heating solution to 121°C to kill microbes
- Using UV light to sterilize bulk product in tanks
- Applying gamma radiation to final packaged product only
Correct Answer: Passing solution through a sterilizing-grade membrane to remove microorganisms
Q38. What is the primary reason gamma irradiation is favored for sterilizing prefilled disposable syringes?
- It is a chemical method that leaves residues
- It provides deep penetration without heat, preserving device integrity
- It is less regulated than steam sterilization
- It requires high humidity for effectiveness
Correct Answer: It provides deep penetration without heat, preserving device integrity
Q39. Which measurement describes the efficiency of HEPA filters in removing particles at the most penetrating particle size?
- 99.97% at 0.3 μm
- 50% at 10 μm
- 100% at 5 μm
- 10% at 0.1 μm
Correct Answer: 99.97% at 0.3 μm
Q40. What is a key limitation of using filtration for sterilizing suspensions with high particulate load?
- Filters increase microbial growth
- Frequent clogging and potential loss of product due to retained solids
- Filters sterilize endotoxins selectively
- Filters cannot be sterilized themselves
Correct Answer: Frequent clogging and potential loss of product due to retained solids
Q41. Which sterilization indicator type provides an immediate visual change but does not prove microbial inactivation?
- Biological indicator culture result
- Chemical integrator
- Product sterility test
- Microbial count reduction assay
Correct Answer: Chemical integrator
Q42. Which physical method is recommended for sterilizing heat-stable surgical instruments?
- Membrane filtration
- Autoclaving (steam sterilization)
- UV irradiation only
- Gamma irradiation for reusable instruments
Correct Answer: Autoclaving (steam sterilization)
Q43. Which process parameter is most relevant when using electron beam (e-beam) radiation for sterilization?
- Pore size selection of filters
- Radiation dose (kGy) and penetration depth
- Steam quality and vacuum level
- Ultraviolet wavelength selection
Correct Answer: Radiation dose (kGy) and penetration depth
Q44. What is the main reason autoclave tape is not sufficient to confirm sterility?
- Tape changes color only with pressure, not temperature
- Tape indicates exposure to heat but does not indicate microbial kill
- Tape cannot survive autoclave conditions
- Tape is only used for labeling and has no indicator properties
Correct Answer: Tape indicates exposure to heat but does not indicate microbial kill
Q45. Which physical sterilization method causes microbial cell death primarily by oxidation of cellular components?
- Moist heat sterilization
- Dry heat sterilization
- Membrane filtration
- UV-C irradiation
Correct Answer: Dry heat sterilization
Q46. Which parameter is most commonly monitored during validation of a sterilization cycle to ensure adequate exposure?
- Ambient room humidity
- Temperature and time profiles recorded during the cycle
- Operator’s checklist completion
- Color of the load packaging
Correct Answer: Temperature and time profiles recorded during the cycle
Q47. Which of the following best describes ‘depth filters’ used in pharmaceutical processing?
- Thin membrane with uniform pores for final sterilization
- Thick porous matrix that traps particles throughout its depth for prefiltration
- Only effective for gas sterilization
- Special filters that sterilize by chemical release
Correct Answer: Thick porous matrix that traps particles throughout its depth for prefiltration
Q48. For sterile processing of ophthalmic eye drops, the recommended physical sterilization approach is:
- Autoclaving after filling in final container
- Terminal sterilization by gamma irradiation after bottling
- Sterile filtration before aseptic filling
- Dry heat depyrogenation of final product
Correct Answer: Sterile filtration before aseptic filling
Q49. Which physical sterilization validation requires incubation to determine the survival of a resistant microorganism?
- Chemical indicator strip
- Biological indicator testing
- Temperature data logger readout
- Bubble point integrity test
Correct Answer: Biological indicator testing
Q50. Which term describes the practice of using a combination of physical controls (temperature/time) to achieve validated sterilization results?
- Sterile filtration mapping
- Sterilization cycle development and validation
- Filter integrity certification only
- UV shadow mapping
Correct Answer: Sterilization cycle development and validation
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