Sterilization and Disinfection — MCQs

Sterilization and Disinfection Indian Medical PG Practice Questions and MCQs

Question 191: Which of the following is NOT a true statement regarding disinfectants?

A. Gluteraldehyde is sporicidal.
B. Hypochlorites are virucidal.
C. Ethylene oxide is an intermediate disinfectant. (Correct Answer)
D. All of the above statements are true.

Explanation: ### Explanation The correct answer is **C**, as Ethylene Oxide (EtO) is classified as a **high-level disinfectant/sterilant**, not an intermediate one. **1. Why Option C is the correct (False) statement:** Disinfectants are categorized into three levels based on their biocidal activity: * **High-level:** Kill all microbes, including bacterial spores (e.g., Glutaraldehyde, Ethylene Oxide, Hydrogen Peroxide). * **Intermediate-level:** Kill mycobacteria, vegetative bacteria, and most viruses/fungi, but **not** bacterial spores (e.g., Alcohols, Iodophors). * **Low-level:** Kill most vegetative bacteria and enveloped viruses (e.g., Quaternary ammonium compounds). **Ethylene Oxide** is a potent alkylating agent used for gaseous sterilization of heat-sensitive items (like heart-lung machines and catheters). It is highly effective against all microorganisms, including spores, making it a sterilant/high-level disinfectant. **2. Analysis of other options:** * **Option A (Glutaraldehyde is sporicidal):** This is **True**. 2% Glutaraldehyde (Cidex) is a high-level disinfectant. It requires 20 minutes for disinfection but 10 hours of immersion to be truly sporicidal ("cold sterilization"). * **Option B (Hypochlorites are virucidal):** This is **True**. Sodium hypochlorite is highly effective against viruses, including HIV and Hepatitis B. It is the disinfectant of choice for cleaning blood spills. **High-Yield Clinical Pearls for NEET-PG:** * **Glutaraldehyde (2%):** Used for endoscopes and bronchoscopes. Once activated, it has a shelf life of 14 days. * **Ethylene Oxide (EtO):** Best for plastic/rubber goods that cannot withstand heat. Its main drawback is toxicity and the requirement for long aeration periods. * **Blood Spills:** Use 1% Hypochlorite for small spills and 10% for large spills. * **Prions:** Most resistant to sterilization; require autoclaving at 134°C for 18 minutes or 1N NaOH for 1 hour.

Question 192: Which of the following agents is used for the sterilization of fiber optic instruments after endoscopy?

A. Glutaraldehyde (Correct Answer)
B. Chlorine
C. Autoclave
D. Phenol

Explanation: **Explanation:** **1. Why Glutaraldehyde is correct:** Fiber optic instruments (like endoscopes, bronchoscopes, and cystoscopes) are **heat-sensitive** and delicate. They cannot withstand the high temperatures of an autoclave. **2% Glutaraldehyde (Cidex)** is the agent of choice for "cold sterilization." It acts by alkylating amino, carboxyl, and hydroxyl groups, affecting nucleic acid and protein synthesis. It is effective against bacteria, spores, fungi, and viruses. For high-level disinfection, an immersion time of 20 minutes is required, while 10 hours is needed to achieve absolute sterilization (sporicidal action). **2. Why the other options are incorrect:** * **Chlorine:** While a potent disinfectant (used for water and HIV-contaminated blood spills), it is highly corrosive to metals and can damage the delicate components of endoscopes. * **Autoclave:** This uses moist heat (121°C). The high temperature and pressure would melt the adhesives and damage the fiber optic bundles of the endoscope. * **Phenol:** Phenolics are protoplasmic poisons used primarily for disinfecting surfaces, floors, and hospital waste. They are too toxic for instruments that come into direct contact with mucous membranes and are not sporicidal. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cidex Stability:** Once activated by adding alkalinizing agents, 2% Glutaraldehyde remains effective for **14 days**. * **Ortho-phthalaldehyde (OPA):** A newer alternative to glutaraldehyde; it is more stable, faster-acting, and does not require activation, though it is more expensive. * **Prions:** Glutaraldehyde is **ineffective** against prions; in fact, it may "fix" them to the instrument. * **Safety:** Glutaraldehyde fumes can cause respiratory irritation and dermatitis; hence, instruments must be thoroughly rinsed with sterile water after immersion.

Question 193: What is the definition of asepsis?

A. Absence of pathogenic microbes (Correct Answer)
B. Disinfection of the surface
C. Prevention of infection
D. Destroying all forms of micro-organisms

Explanation: **Explanation:** **Asepsis** refers to a state or condition where **pathogenic microorganisms are absent**. In a clinical setting, aseptic techniques are employed to prevent the introduction of pathogens into susceptible sites (like surgical wounds) by ensuring that the environment, instruments, and personnel are free from disease-causing agents. **Analysis of Options:** * **Option A (Correct):** Asepsis literally means "without sepsis." It focuses on the **absence** of pathogens to prevent contamination. * **Option B (Incorrect):** This describes **Disinfection**, which is the process of reducing the number of viable microorganisms on inanimate surfaces to a level that is not harmful to health. * **Option C (Incorrect):** This is the definition of **Antisepsis**. While related, antisepsis specifically refers to the use of chemical agents (antiseptics) on **living tissue** to inhibit or destroy pathogens to prevent infection. * **Option D (Incorrect):** This defines **Sterilization**, which is the absolute destruction of **all** forms of microbial life, including highly resistant bacterial spores. **High-Yield NEET-PG Pearls:** * **Asepsis vs. Antisepsis:** Asepsis is a *condition* (absence of pathogens), while antisepsis is a *process* (applying chemicals to skin/tissue). * **Father of Antiseptic Surgery:** Joseph Lister (used carbolic acid/phenol). * **Critical Concept:** Sterilization kills **spores**; disinfection generally does not. * **Standard Precautions:** These are the modern foundation of asepsis in hospitals, treating all blood and body fluids as potentially infectious.

Question 194: The biological indicator used in various autoclave machines for quality check is:

A. Clostridium tetani
B. Bacillus stereothermophilus (Correct Answer)
C. Bacillus pumilus
D. Bacillus subtilis

Explanation: ### Explanation **1. Why Bacillus stearothermophilus is Correct:** Autoclaving (moist heat sterilization) works by denaturing proteins and enzymes at high temperatures (typically 121°C for 15 minutes). **Geobacillus (formerly Bacillus) stearothermophilus** is used as the gold standard biological indicator because it is a thermophilic (heat-loving) spore-former. Its spores are highly resistant to moist heat, with a **D-value** (time required to kill 90% of the population) specifically calibrated for autoclave temperatures. If the autoclave cycle can kill these highly resistant spores, it is assumed that all other pathogenic vegetative bacteria and spores have been destroyed. **2. Analysis of Incorrect Options:** * **Clostridium tetani (A):** While it forms spores, it is a human pathogen and not standardized for sterilization monitoring. It is also less heat-resistant than thermophilic bacilli. * **Bacillus pumilus (C):** This is the biological indicator used for **Ionizing Radiation** (Gamma rays) sterilization. * **Bacillus subtilis (D):** Specifically the *globigii* or *niger* strains, these are used as biological indicators for **Dry Heat Sterilization** (Hot Air Oven) and **Ethylene Oxide (ETO)** gas sterilization. **3. High-Yield Clinical Pearls for NEET-PG:** * **Indicator for Glutaraldehyde (Cidex):** *Bacillus atrophaeus*. * **Indicator for Plasma Sterilization:** *Bacillus stearothermophilus*. * **Chemical Indicator:** **Browne’s tubes** (color change from red to green) or **Bowie-Dick test** (specifically for vacuum-assisted autoclaves to check for air leaks). * **Sterilization Check:** Biological indicators are the only method that confirms the actual "killing" of microorganisms, whereas chemical indicators only confirm that the required temperature/pressure was reached.

Question 195: What is the typical temperature of an autoclave operating at 30 psi of pressure?

A. 121°C
B. 100°C
C. 63°C
D. 134°C (Correct Answer)

Explanation: ### Explanation The autoclave operates on the principle of **moist heat sterilization** using saturated steam under pressure. The core concept is that increasing the pressure within a closed vessel raises the boiling point of water, allowing steam to reach temperatures higher than 100°C, which is necessary to kill highly resistant bacterial spores. **Why 134°C is correct:** Standard autoclaving typically follows two main protocols: 1. **15 psi at 121°C for 15–20 minutes:** This is the standard cycle for most laboratory media and surgical instruments. 2. **30 psi at 134°C for 3 minutes:** Known as "Flash Sterilization" or "High-speed sterilization," this higher pressure allows for a significantly higher temperature, reducing the required holding time. This is commonly used for unwrapped instruments in emergency clinical settings. **Analysis of Incorrect Options:** * **A. 121°C:** This is the temperature achieved at **15 psi**. While it is the most common autoclave setting, it does not correspond to 30 psi. * **B. 100°C:** This is the boiling point of water at sea level. Methods using 100°C include Tyndallization and steaming (Koch’s sterilizer), which are not pressurized. * **C. 63°C:** This temperature is used in **Holder Pasteurization** (63°C for 30 minutes) for milk, which is a disinfection process, not sterilization. **High-Yield Clinical Pearls for NEET-PG:** * **Sterilization Check:** The biological indicator of choice for autoclaving is **_Geobacillus stearothermophilus_** spores. * **Chemical Indicator:** **Browne’s tubes** (color change from red to green) or **Bowie-Dick test** (for vacuum efficiency). * **Mechanism of Action:** Moist heat kills microorganisms by **denaturation and coagulation of structural proteins and enzymes**. * **Prion Protocol:** For prion-contaminated instruments, autoclaving at **134°C for 1 hour** is recommended.

Question 196: Waste in yellow plastic bags are treated by which method?

A. Incineration (Correct Answer)
B. Autoclaving
C. Microwaving
D. Shredding

Explanation: **Explanation:** The management of Biomedical Waste (BMW) is governed by specific color-coding guidelines. **Yellow bags** are designated for highly infectious, non-recyclable waste, including human anatomical waste (tissues/organs), animal waste, soiled waste (blood-soaked cotton/dressings), and discarded medicines. **1. Why Incineration is Correct:** Incineration is a high-temperature dry oxidation process that reduces organic and combustible waste into inorganic, incombustible matter (ash). It is the preferred method for yellow bag waste because it ensures the complete destruction of pathogens and anatomical structures, significantly reducing the volume of waste. For anatomical waste, incineration is the gold standard to prevent aesthetic and public health concerns. **2. Why Other Options are Incorrect:** * **Autoclaving (Option B):** This uses pressurized steam (moist heat). It is the primary treatment for **Red bags** (recyclable plastic waste like catheters/tubing) to ensure sterilization before recycling. It is not suitable for anatomical waste as it doesn't reduce volume or destroy the specimen's identity. * **Microwaving (Option C):** Similar to autoclaving, it is used for disinfecting infectious waste (Red bag) but is contraindicated for anatomical, cytotoxic, or pharmaceutical waste. * **Shredding (Option D):** This is a secondary process used to make waste unrecognizable and prevent reuse. It is typically performed *after* disinfection (autoclaving/microwaving), not as a primary treatment for yellow bag contents. **High-Yield Clinical Pearls for NEET-PG:** * **Yellow Bag:** Anatomical waste, soiled waste, chemical waste, and discarded medicines. (Mnemonic: **Y**ellow = **Y**ucky/Anatomical). * **Red Bag:** Recyclable contaminated waste (tubing, bottles, gloves). (Mnemonic: **R**ed = **R**ecyclable). * **White (Translucent) Container:** Sharps (needles, scalpels). Treated by Autoclaving + Shredding. * **Blue Box:** Glassware and metallic body implants. * **Note:** Chlorinated plastic bags should **not** be incinerated due to the risk of releasing toxic dioxins and furans.

Question 197: Which of the following is an important disinfectant on account of effectively destroying Gram-positive and Gram-negative bacteria, viruses, and even spores at low pH levels?

A. Phenol
B. Alcohol
C. Chlorine (Correct Answer)
D. Hexachlorophene

Explanation: **Explanation:** **Chlorine** is a potent oxidizing agent and a high-level disinfectant. Its efficacy is primarily due to the formation of **hypochlorous acid (HOCl)** when dissolved in water. At a **low pH (acidic environment)**, the dissociation of hypochlorous acid is suppressed, allowing it to remain in its most active, undissociated form. This form easily penetrates microbial cell walls, effectively destroying Gram-positive and Gram-negative bacteria, viruses (including HBV and HIV), and even bacterial **spores** (at higher concentrations and longer contact times). **Analysis of Incorrect Options:** * **Phenol:** It is a protoplasmic poison that acts by denaturing proteins and disrupting cell membranes. While it is effective against vegetative bacteria, it is **not sporicidal** and has poor activity against many viruses. * **Alcohol (e.g., Ethanol/Isopropanol):** These act by denaturing proteins and dissolving lipids. They are effective against vegetative bacteria and enveloped viruses but are **not sporicidal** and cannot be used for sterilization. * **Hexachlorophene:** This is a chlorinated bisphenol. It is highly effective against Gram-positive bacteria (especially Staphylococci) but has **weak activity** against Gram-negative bacteria, fungi, and viruses, and is completely **non-sporicidal**. **Clinical Pearls for NEET-PG:** * **Mechanism:** Chlorine acts by oxidation of sulfhydryl (-SH) groups of essential enzymes. * **pH Dependency:** Chlorine is most active at an acidic pH; its activity decreases significantly as the pH rises (alkaline). * **Organic Matter:** Chlorine is easily inactivated by organic matter (blood, pus, feces); hence, surfaces must be cleaned before application. * **Standard Usage:** Sodium hypochlorite (1% or 10,000 ppm) is the disinfectant of choice for **blood spills** (e.g., HIV/HBV contamination).

Question 198: Which of the following represents the most appropriate method for hand hygiene in a healthcare setting?

A. Cleaning hands with an alcohol-based hand rub before and after examining each patient. (Correct Answer)
B. Washing hands with soap and water when there is visible blood on the hands.
C. Using soap and Dettol exclusively for hand washing prior to surgery.
D. Hand rubbing with alcohol-based solutions is never a substitute for hand washing.

Explanation: ### Explanation **1. Why Option A is Correct:** According to the **WHO Five Moments for Hand Hygiene**, alcohol-based hand rubs (ABHR) are the **gold standard** for routine decontamination in clinical settings. ABHR is preferred because it is faster, more effective against a broad spectrum of microorganisms (except spores), and better tolerated by the skin compared to repeated soap and water washing. It should be performed before and after direct patient contact to prevent the cross-transmission of pathogens. **2. Analysis of Incorrect Options:** * **Option B:** While washing hands with soap and water is mandatory when hands are visibly soiled with blood or body fluids, this option is a *specific scenario* rather than the most appropriate *general method* for routine hygiene. * **Option C:** Pre-surgical hand preparation requires a surgical hand scrub (e.g., Povidone-iodine or Chlorhexidine) for a specific duration (2–6 minutes). Dettol (Chloroxylenol) is not the primary agent used for surgical antisepsis in modern evidence-based practice. * **Option D:** This is factually incorrect. ABHR is the **primary substitute** for hand washing when hands are not visibly soiled. **3. High-Yield NEET-PG Pearls:** * **Contact Time:** For ABHR, the recommended time is **20–30 seconds**; for hand washing with soap and water, it is **40–60 seconds**. * **The "Spore" Exception:** ABHR is **ineffective** against *Clostridium difficile* spores and Non-enveloped viruses (e.g., Norovirus). In these cases, soap and water must be used. * **Composition:** Effective ABHR typically contains 60–80% Ethanol or Isopropanol. * **Resident vs. Transient Flora:** Hand hygiene primarily aims to remove **transient flora** (e.g., *S. aureus*, Gram-negative bacilli) acquired during patient contact.

Question 199: A known HIV-positive patient admitted in an isolation ward after abdominal surgery following an accident. The resident doctor who changed his dressing the next day found it to be soaked in blood. Which of the following would be the right method of choice for discarding the dressing?

A. Pour 1% hypochlorite on the dressing material and send it for incineration in an appropriate bag.
B. Pour 5% hypochlorite on the dressing material and send it for incineration in an appropriate bag.
C. Put the dressing material directly in an appropriate bag and send for incineration. (Correct Answer)
D. Pour 2% Lysol on the dressing material and send it for incineration in an appropriate bag.

Explanation: ### Explanation The management of biomedical waste (BMW) is governed by the **Biomedical Waste Management Rules (2016)**. According to these guidelines, items contaminated with blood and body fluids (soiled waste), such as dressings, cotton swabs, and plaster casts, are categorized as **Yellow Bag Waste**. **1. Why Option C is Correct:** The standard protocol for soiled waste is to place it directly into a non-chlorinated yellow plastic bag for **incineration** or plasma pyrolysis. Pre-treatment with chemical disinfectants (like hypochlorite) at the site of generation is **no longer recommended** for solid soiled waste. This is because: * Incineration at high temperatures effectively destroys all pathogens, including HIV. * Pre-treating with chlorine-based compounds can lead to the release of toxic **dioxins and furans** during the incineration process. **2. Why Other Options are Incorrect:** * **Options A & B:** While hypochlorite is an excellent disinfectant for blood spills on floors or surfaces, it is contraindicated for waste destined for incineration due to environmental hazards (dioxin release). * **Option D:** Lysol (quaternary ammonium/phenolic) is used for surface disinfection but is not a substitute for the mandated disposal protocol of soiled dressings. **3. Clinical Pearls for NEET-PG:** * **Yellow Bag:** Includes soiled waste, anatomical waste, expired medicines, and chemical liquid waste. * **Red Bag:** For recyclable plastic waste (tubing, bottles, syringes without needles). These are autoclaved/microwaved, not incinerated. * **White Container (Puncture-proof):** For sharps (needles, scalpels). * **Blue Box/Cardboard:** For glass vials and metallic implants. * **HIV Disinfection:** HIV is a fragile virus. For **surface spills**, 1% Sodium Hypochlorite (contact time 20–30 mins) is the gold standard. For large spills (>10ml), 10% hypochlorite is used.

Question 200: Which of the following statements regarding the properties of common disinfectants is FALSE?

A. Glutaraldehyde is sporicidal.
B. Hypochlorites are viricidal.
C. Alcohol is fungicidal and bactericidal but not sporicidal.
D. Phenol usually requires organic matter to act. (Correct Answer)

Explanation: ### Explanation **1. Why Option D is the correct (False) statement:** The statement is false because **phenols are actually notable for being effective even in the presence of organic matter** (such as pus, blood, or feces). Unlike many other disinfectants (like halogens) that are inactivated by organic debris, phenolic compounds (e.g., Lysol, Cresol) maintain their efficacy. This property makes them particularly useful for disinfecting excreta and surfaces in clinical settings. **2. Analysis of Incorrect Options (True Statements):** * **A. Glutaraldehyde is sporicidal:** This is true. Glutaraldehyde (e.g., Cidex) is a high-level disinfectant. It is sporicidal after prolonged exposure (usually 10 hours) and is commonly used for "cold sterilization" of endoscopes. * **B. Hypochlorites are viricidal:** This is true. Hypochlorites (Bleach) are potent oxidizing agents effective against a wide range of viruses, including HBV and HIV. They are the disinfectant of choice for managing blood spills. * **C. Alcohol is fungicidal and bactericidal but not sporicidal:** This is true. 70% Ethyl or Isopropyl alcohol acts by denaturing proteins. While effective against vegetative bacteria, fungi, and enveloped viruses, it lacks the ability to penetrate and kill bacterial spores. **3. High-Yield Clinical Pearls for NEET-PG:** * **Chick-Martin Test:** Used to determine the efficacy of disinfectants in the presence of organic matter (specifically using sterilized feces). * **Rideal-Walker Coefficient:** Compares a disinfectant's efficiency to Phenol (Phenol Coefficient). * **Endoscope Disinfection:** 2% Glutaraldehyde is the standard; however, **Ortho-phthalaldehyde (OPA)** is a newer alternative that is faster and less irritating. * **Blood Spills:** Sodium hypochlorite (1% for small spills, 10% for large spills) is the gold standard.

Practice by Chapter

Physical Methods of Sterilization

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Chemical Methods of Sterilization

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Disinfectants and Antiseptics

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Sterilization Monitoring

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Hospital Sterilization Protocols

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Surgical Instruments Sterilization

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Endoscope Reprocessing

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Biological Indicators

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Factors Affecting Sterilization

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Quality Control in Sterilization

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Biofilm and Its Implications

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Regulatory Aspects of Sterilization

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