Sterilization and Disinfection — MCQs

Sterilization and Disinfection Indian Medical PG Practice Questions and MCQs

Question 201: Which of the following is best for sterilizing heat-labile solutions?

A. Dry heat
B. Autoclave
C. Membrane filtration (Correct Answer)
D. Pasteurization

Explanation: **Explanation:** **Why Membrane Filtration is Correct:** Sterilization of **heat-labile (heat-sensitive)** solutions requires a method that does not involve high temperatures, which would otherwise denature proteins or degrade chemical compounds. Membrane filtration is a **physical method** that removes microorganisms (bacteria, fungi, and some large viruses) by passing the liquid through pores smaller than the microbes (typically **0.22 µm**). This is the gold standard for sterilizing sera, antibiotic solutions, vaccines, and enzyme preparations. **Why Other Options are Incorrect:** * **Dry Heat (Hot Air Oven):** Uses high temperatures (160°C for 2 hours). It is used for glassware and oil-based substances but would destroy heat-sensitive liquids. * **Autoclave (Moist Heat):** Uses saturated steam under pressure (121°C for 15 mins). While highly effective for surgical instruments and culture media, it causes irreversible damage to heat-labile proteins and vitamins. * **Pasteurization:** This is a **disinfection** process, not sterilization. It uses moderate heat to reduce the microbial load in milk and beverages but does not kill bacterial spores. **High-Yield Clinical Pearls for NEET-PG:** * **Pore Size:** A 0.22 µm filter removes most bacteria, but to remove smaller viruses or *Mycoplasma*, even smaller pore sizes are required. * **HEPA Filters:** A form of filtration used for **air sterilization** in OTs and laminar airflow hoods (removes particles >0.3 µm). * **Ethylene Oxide (EtO):** If the question asks for heat-labile **equipment** (like plastics or heart-lung machines) rather than solutions, EtO is the preferred gaseous sterilant. * **Flash Sterilization:** A rapid autoclaving method (134°C for 3 mins) used for urgently needed surgical instruments.

Question 202: What is the best method for sterilizing disposable syringes?

A. Hot air oven
B. Ultraviolet rays
C. Gamma rays (Correct Answer)
D. Boiling

Explanation: **Explanation:** **Gamma rays** are the preferred method for sterilizing disposable plastic items like syringes, catheters, and sutures. This process, often referred to as **"Cold Sterilization,"** utilizes ionizing radiation (Cobalt-60) to destroy microbial DNA without generating heat. Since disposable syringes are made of heat-sensitive plastics (like polypropylene), they cannot withstand high temperatures, making gamma radiation the gold standard for industrial-scale sterilization of pre-packed medical devices. **Why other options are incorrect:** * **Hot air oven (Dry Heat):** Operates at 160°C for 1 hour. This temperature would melt or deform the plastic components of disposable syringes. It is used for glassware and metallic instruments. * **Ultraviolet (UV) rays:** These are non-ionizing radiations with poor penetrating power. They are used for surface disinfection or air sterilization in OTs but cannot penetrate the packaging or the interior of a syringe. * **Boiling:** This is a method of high-level disinfection, not sterilization, as it fails to kill bacterial spores. Furthermore, repeated boiling can damage plastic. **High-Yield Clinical Pearls for NEET-PG:** * **Cold Sterilization:** Refers to sterilization by ionizing radiation (Gamma rays) or chemicals (Glutaraldehyde), where no heat is applied. * **Ethylene Oxide (EtO):** Another common method for heat-sensitive items (like heart-lung machines), but Gamma rays are preferred for bulk-manufactured disposables due to better penetration and no toxic residue. * **Dosage:** The standard dose of Gamma radiation for sterilization is **2.5 megarads**.

Question 203: Plasma sterilization is done by

A. UV rays
B. Ethylene oxide
C. Hydrogen peroxide (Correct Answer)
D. Gamma rays

Explanation: ***Hydrogen peroxide*** - Plasma sterilization uses **hydrogen peroxide vapor** that is converted into plasma state by electromagnetic energy (radiofrequency or microwave) - The plasma contains **highly reactive free radicals** (hydroxyl and hydroperoxyl) that destroy microorganisms by disrupting cell membranes and DNA - Common system: **STERRAD** (Sterilization by Reactive Gas Discharge) - Advantages: **Low temperature** process (suitable for heat-sensitive instruments), **non-toxic**, leaves no residue, and has a **short cycle time** *UV rays (Incorrect)* - Used for surface disinfection and air sterilization, not plasma sterilization - Limited penetration power *Ethylene oxide (Incorrect)* - Gas sterilization method, not plasma-based - Requires longer aeration time due to toxicity *Gamma rays (Incorrect)* - Ionizing radiation sterilization method - Used for industrial sterilization, not plasma sterilization

Question 204: Autoclaving is a method of sterilization. The process involves heating by steam at

A. 10 lb pressure, 100 °C temperature for 10 minutes
B. 20 lb pressure, 120 °C temperature for 20 minutes
C. 15 lb pressure, 121 °C temperature for 15 minutes (Correct Answer)
D. 25 lb pressure, 120 °C temperature for 25 minutes

Explanation: ***15 lb pressure, 121 °C temperature for 15 minutes*** - This is the **standard recommended parameter** for routine sterilization in an autoclave, ensuring the destruction of most pathogenic and non-pathogenic microorganisms, including spores. - The combination of **steam at 121°C** (equivalent to 15 psi) for at least 15 minutes provides efficient heat transfer and penetration required for effective sterilization. *10 lb pressure, 100 °C temperature for 10 minutes* - This temperature (100 °C) is used for **boiling** or pasteurization, which is generally not sufficient for complete sterilization as it may not kill highly resistant spores. - The **lower pressure and temperature** would likely not achieve the required sterility assurance level. *20 lb pressure, 120 °C temperature for 20 minutes* - While higher pressure can achieve sterilization, this is **not the standard parameter** used for typical autoclave sterilization. - The standard combination of 15 lb at 121°C is more commonly recommended and taught in medical curricula. *25 lb pressure, 120 °C temperature for 25 minutes* - This combination of **higher pressure and longer duration** would likely achieve sterilization, but it exceeds the minimum standard parameters. - While effective, it is not the most **efficient or commonly recommended** setting and could potentially lead to material degradation for some items.

Question 205: Tetanus spores can be effectively killed by

A. Gamma irradiation (Correct Answer)
B. Anti tetanus Serum
C. Tetanus toxoid
D. Large doses of penicillin

Explanation: ***Gamma irradiation*** - **Gamma irradiation** denatures proteins and damages nucleic acids, effectively destroying bacterial spores like those of *Clostridium tetani*. - This method is used for sterilizing heat-sensitive materials and achieving a high level of **microbial inactivation**. *Anti tetanus Serum* - **Anti-tetanus serum (ATS)** contains pre-formed antibodies that neutralize the circulating tetanus **toxin**, not the spores. - ATS is used for **passive immunization** and treatment of tetanus, but it does not kill the bacterial spores themselves. *Tetanus toxoid* - **Tetanus toxoid** is an inactivated bacterial toxin used to stimulate an immune response and provide **active immunization** against the toxin. - It does not directly kill tetanus spores; instead, it prevents the effects of the toxin produced by the spores. *Large doses of penicillin* - **Penicillin** is an antibiotic that can kill *Clostridium tetani* bacteria, but it is much less effective against the dormant and highly resistant **tetanus spores**. - While it can help eradicate active infection, spores often require more potent physical or chemical sterilization methods.

Question 206: Which of the following represent the properties of an ideal disinfectant? 1. It is broad spectrum 2. It is fast acting 3. It is non-toxic Select the correct answer using the code given below:

A. 1 and 3 only
B. 1, 2 and 3 (Correct Answer)
C. 2 and 3 only
D. 1 and 2 only

Explanation: ***Correct Answer: 1, 2 and 3*** An **ideal disinfectant** should possess all three properties: - **Broad spectrum activity** - It should effectively kill or inactivate a wide range of microorganisms, including bacteria, viruses, fungi, and spores - **Fast-acting** - It should achieve rapid antimicrobial effect after application, which is crucial in clinical settings to prevent pathogen spread and minimize exposure time - **Non-toxic** - It should be safe for humans and animals when used as directed, minimizing adverse effects on users, patients, and the environment Additional desirable properties include stability, solubility in water, non-corrosive nature, pleasant odor, and cost-effectiveness. *Incorrect: 1 and 3 only* - While broad spectrum and non-toxic nature are crucial, **fast action** is also an essential property for an ideal disinfectant to ensure rapid microbial elimination - A disinfectant that is not fast-acting would be less practical in situations requiring quick decontamination *Incorrect: 2 and 3 only* - A disinfectant that is fast-acting and non-toxic is insufficient without **broad-spectrum activity** - Without broad-spectrum activity, it may fail to eliminate certain pathogens, limiting its overall utility in infection control *Incorrect: 1 and 2 only* - Although broad spectrum and fast-acting properties are important, an ideal disinfectant must also be **non-toxic** - Toxicity would severely limit its application, especially in healthcare settings where human contact is frequent

Question 207: Biological indicator for determining efficacy of autoclaving is

A. Bacillus stearothermophilus (Correct Answer)
B. Pseudomonas aeruginosa
C. Clostridium perfringens
D. Salmonella typhi

Explanation: ***Bacillus stearothermophilus*** - *Bacillus stearothermophilus* (now *Geobacillus stearothermophilus*) is a **thermoduric spore-forming bacterium** used as a biological indicator for **autoclave efficacy**. - Its spores are highly **resistant to heat**, making them ideal for challenging the sterilization process. *Pseudomonas aeruginosa* - *Pseudomonas aeruginosa* is a **Gram-negative bacterium** known for causing hospital-acquired infections, but it is **not used as a biological indicator** for autoclaving. - It is **less resistant to heat** and sterilization methods compared to bacterial spores. *Clostridium perfringens* - *Clostridium perfringens* is a **spore-forming anaerobic bacterium** associated with gas gangrene and food poisoning. - While it forms spores, its **heat resistance profile is different** from that of *Bacillus stearothermophilus*, and it is not the standard biological indicator for autoclaving. *Salmonella typhi* - *Salmonella typhi* is a **Gram-negative bacterium** that causes typhoid fever. - It is a **pathogen but not a spore-former**, and therefore, it is easily killed by autoclaving and not suitable as a biological indicator for monitoring sterilization effectiveness.

Question 208: Which is false regarding Spaulding's criteria?

A. Non critical items require only decontamination
B. Cardiac catheters are examples of critical items
C. Semi critical items need low level disinfection (Correct Answer)
D. Semi critical items are those which come in contact with mucous membrane or non intact skin

Explanation: ***Semi critical items need low level disinfection*** - This statement is **FALSE** and is the **correct answer** to this question. - **Semi-critical items** require **high-level disinfection**, NOT low-level disinfection. - Semi-critical items come into contact with mucous membranes or non-intact skin and require removal of all vegetative bacteria, fungi, mycobacteria, and most viruses. - Examples include endoscopes, laryngoscope blades, and respiratory therapy equipment. *Non critical items require only decontamination* - This statement is **TRUE** (or at least acceptable in context). - Non-critical items contact intact skin and require **cleaning** and **low-level disinfection** (which falls under the umbrella term "decontamination"). - Examples include blood pressure cuffs, stethoscopes, and bedpans. *Cardiac catheters are examples of critical items* - This statement is **TRUE**. - **Cardiac catheters** enter the **vascular system** (sterile tissue), making them **critical items**. - Critical items require **sterilization** to prevent severe systemic infection. *Semi critical items are those which come in contact with mucous membrane or non intact skin* - This statement is **TRUE** and correctly defines **semi-critical items** according to Spaulding's classification. - This is the standard definition used in medical device processing protocols.

Question 209: Browne's tube is used for -

A. Radiation
B. Steam sterilization (Correct Answer)
C. Filtration
D. Chemical sterilization

Explanation: ***Steam sterilization*** - **Browne's tubes** are chemical indicators used to monitor the effectiveness of **steam sterilization** processes. - They contain a **temperature-sensitive chemical** that changes color when exposed to specific temperatures and steam for a sufficient duration, indicating proper sterilization parameters have been met. *Radiation* - **Radiation sterilization** typically uses methods like **gamma radiation** or **electron beam radiation** to kill microorganisms. - Browne's tubes are not designed or used to monitor the efficacy of these radiation-based sterilization methods. *Filtration* - **Filtration** is a method used to remove microorganisms from liquids or gases, often without killing them, by trapping them in a filter medium. - This process does not involve heat or steam, and Browne's tubes have no role in monitoring filtration efficacy. *Chemical sterilization* - **Chemical sterilization** uses chemical agents like **ethylene oxide** or **hydrogen peroxide plasma** to sterilize heat-sensitive instruments. - While chemical indicators exist for these methods, Browne's tubes are specifically formulated for and calibrated to respond to the parameters of steam sterilization.

Question 210: Disinfection by autoclaving is NOT recommended for -

A. Sharp instrument (Correct Answer)
B. Linen
C. Sputum
D. Glass syringes

Explanation: ***Sharp instrument*** - While **stainless steel sharp instruments** can technically withstand autoclaving, repeated exposure to **high heat** and **steam** under pressure can lead to the dulling or corrosion of delicate cutting edges over time. - This is particularly true for **carbon steel blades** and instruments not made from high-grade stainless steel. - Alternative sterilization methods like **chemical sterilization** or **low-temperature sterilization** may be preferred to preserve the **sharpness** and extend the functional lifespan of precision cutting instruments. - In practice, disposable sharp instruments are increasingly preferred to avoid this issue. *Linen* - **Autoclaving** is highly effective and commonly used for sterilizing linen materials, such as surgical gowns, drapes, and towels, as it ensures the destruction of microorganisms. - The heat and moisture penetrate the fabric efficiently, making it a reliable method for rendering textiles **pathogen-free**. *Sputum* - Autoclaving is an appropriate method for **sterilizing biological waste** contaminated with infectious agents, including sputum. - It effectively **decontaminates** the material before its disposal, preventing the spread of pathogens. *Glass syringes* - **Glass syringes** are resistant to the high temperatures and pressures of autoclaving, making it a suitable method for their sterilization. - The process ensures that the syringes are **sterile** and safe for reuse in medical procedures, provided they are designed for such repeated sterilization.

Practice by Chapter

Physical Methods of Sterilization

Practice Questions

Chemical Methods of Sterilization

Practice Questions

Disinfectants and Antiseptics

Practice Questions

Sterilization Monitoring

Practice Questions

Hospital Sterilization Protocols

Practice Questions

Surgical Instruments Sterilization

Practice Questions

Endoscope Reprocessing

Practice Questions

Biological Indicators

Practice Questions

Factors Affecting Sterilization

Practice Questions

Quality Control in Sterilization

Practice Questions

Biofilm and Its Implications

Practice Questions

Regulatory Aspects of Sterilization

Practice Questions

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