Sterilization and Disinfection — MCQs

Sterilization and Disinfection Indian Medical PG Practice Questions and MCQs

Question 71: The Rideal-Walker test measures the efficacy of disinfection by determining which of the following?

A. Phenol coefficient (Correct Answer)
B. Glutaraldehyde coefficient
C. Ethanal coefficient
D. Formalin coefficient

Explanation: **Explanation:** The **Rideal-Walker (RW) test** is a standardized laboratory method used to evaluate the germicidal efficiency of a disinfectant. **Why the correct answer is right:** The test determines the **Phenol Coefficient**, which is a numerical value representing the killing power of a disinfectant compared to pure phenol under identical conditions. In this test, a specific organism (usually *Salmonella typhi*) is exposed to varying dilutions of the test disinfectant and phenol. The coefficient is calculated by dividing the highest dilution of the disinfectant that kills the organism in 7.5 minutes (but not in 5 minutes) by the corresponding dilution of phenol. A coefficient >1 indicates the disinfectant is more effective than phenol. **Why the incorrect options are wrong:** * **B, C, and D:** While Glutaraldehyde, Ethanol (Ethanal is a typo for Ethanol/Acetaldehyde), and Formalin are potent disinfectants/sterilants, they are not used as the "gold standard" reference point in traditional efficacy testing. Phenol was the first widely used antiseptic (introduced by Joseph Lister), making it the historical and scientific benchmark for these calculations. **High-Yield Clinical Pearls for NEET-PG:** * **Chick-Martin Test:** A modification of the RW test that adds organic matter (dried yeast) to better simulate real-world conditions, as organic matter often neutralizes disinfectants. * **Target Organism:** *Salmonella typhi* is the standard organism used in the RW test. * **Limitations:** The RW test only measures bacteriostatic/bactericidal activity against specific vegetative bacteria and does not account for efficacy against spores, viruses, or the presence of organic debris. * **Phenol (Carbolic Acid):** Known as the "Father of Antiseptic Surgery." It acts by disrupting cell membranes and precipitating proteins.

Question 72: What temperature is used for pasteurization?

A. 72°C for 20 minutes
B. 63°C for 30 minutes (Correct Answer)
C. 100°C for 10 minutes
D. 94°C for 20 minutes

Explanation: **Explanation:** Pasteurization is a process of heat treatment used primarily for milk to eliminate pathogenic microorganisms (like *Mycobacterium bovis*, *Brucella*, and *Salmonella*) without significantly altering the nutritional quality or flavor. **1. Why Option B is Correct:** There are two primary methods of pasteurization recognized globally: * **Holder Method (Low-Temperature Holding):** Milk is heated to **63°C (145°F) for 30 minutes**, followed by rapid cooling to below 10°C. This is the method described in Option B. * **Flash Method (High-Temperature Short-Time - HTST):** Milk is heated to **72°C for 15 seconds**, followed by rapid cooling. **2. Analysis of Incorrect Options:** * **Option A (72°C for 20 minutes):** This is incorrect because the 72°C (Flash method) only requires **15 seconds**, not 20 minutes. Prolonged heating at this temperature would denature milk proteins. * **Option C (100°C for 10 minutes):** This describes boiling. While it kills most vegetative bacteria, it is not pasteurization and can alter the taste and composition of milk. * **Option D (94°C for 20 minutes):** This temperature is too high for pasteurization and does not correspond to any standard sterilization protocol. **3. NEET-PG High-Yield Pearls:** * **Target Organism:** Pasteurization is specifically designed to kill *Coxiella burnetii* (the most heat-resistant non-spore-forming pathogen found in milk). If *C. burnetii* is destroyed, all other milk-borne pathogens are assumed dead. * **Efficacy Test:** The **Phosphatase Test** is used to check the efficiency of pasteurization. Since the enzyme phosphatase is destroyed at pasteurization temperatures, its absence indicates successful treatment. * **Note:** Pasteurization **does not kill bacterial spores**; therefore, it is a disinfection process, not sterilization.

Question 73: What type of filters are used to purify air in biological safety cabinets?

A. HEPA (High efficiency particulate air) filters (Correct Answer)
B. Sietz filters
C. Berkfield filters
D. Millipore/Porcelain filters

Explanation: **Explanation:** **Correct Answer: A. HEPA (High efficiency particulate air) filters** HEPA filters are the standard for air purification in Biological Safety Cabinets (BSCs) and laminar airflow systems. They are composed of pleated mats of glass fibers and are designed to remove **99.97% of particles** with a size of **0.3 μm** or larger. This size is critical because 0.3 μm is considered the "most penetrating particle size" (MPPS); particles both larger and smaller are trapped with even higher efficiency through mechanisms like interception, impaction, and diffusion. **Why the other options are incorrect:** * **B. Seitz filters:** These are depth filters made of **asbestos** (or cellulose). They are used for filtering liquids (like toxins or sera) rather than air and are rarely used today due to the carcinogenic nature of asbestos. * **C. Berkefeld filters:** These are made of **Kieselguhr (diatomaceous earth)**. They are used for water purification and are classified based on pore size (W, M, and V), but they are not used for high-efficiency air filtration in laboratory settings. * **D. Millipore/Porcelain filters:** **Millipore** filters are membrane filters made of cellulose esters, used primarily for sterilizing thermolabile liquids. **Porcelain (Chamberland)** filters are made of unglazed porcelain and were historically used to produce "filter-passing" viral filtrates. **High-Yield Clinical Pearls for NEET-PG:** * **Efficiency:** HEPA filters remove bacteria, spores, and fungi, but may not be 100% effective against the smallest individual viruses (though they trap viruses attached to larger droplets). * **Laminar Flow:** HEPA filters provide "laminar" (unidirectional) airflow, which prevents turbulence and cross-contamination. * **Testing:** The integrity of a HEPA filter is tested using the **DOP (Dioctyl Phthalate) test**, which creates a smoke of known particle size to check for leaks. * **BSCs:** Remember that Class II and Class III BSCs utilize HEPA filters for both inflow and exhaust air to protect the personnel, the product, and the environment.

Question 74: Steam under pressure is used in which type of sterilization?

A. Pasteurization
B. Autoclave (Correct Answer)
C. Koch's sterilizer
D. Hot air oven

Explanation: **Explanation:** The correct answer is **Autoclave**. This method utilizes **moist heat sterilization** in the form of **steam under pressure**. **1. Why Autoclave is Correct:** The principle of the autoclave is that water boils when its vapor pressure equals the surrounding atmospheric pressure. By increasing the pressure inside a closed vessel, the boiling point of water rises above 100°C. The standard setting is **121°C at 15 psi for 15 minutes**. This high-temperature steam has greater penetrating power and kills microorganisms, including highly resistant **bacterial spores**, by denaturing and coagulating their structural proteins and enzymes. **2. Why Other Options are Incorrect:** * **Pasteurization:** Uses moist heat at temperatures **below 100°C** (e.g., 63°C for 30 mins or 72°C for 15 secs). It is used for milk and does not kill spores. * **Koch’s Sterilizer:** Uses **steam at atmospheric pressure (100°C)** for 90 minutes or via Tyndallization (intermittent steaming). It is used for media containing gelatin or eggs that would be damaged by higher temperatures. * **Hot Air Oven:** This is a method of **dry heat sterilization** (typically 160°C for 2 hours). It does not use steam; it kills microbes via oxidation of intracellular components. **High-Yield Clinical Pearls for NEET-PG:** * **Sterilization Indicator:** The biological indicator for an autoclave is ***Geobacillus stearothermophilus*** (formerly *Bacillus stearothermophilus*). * **Uses:** Ideal for surgical instruments, dressings, linen, and culture media. * **Flash Sterilization:** A rapid autoclaving method (134°C for 3 minutes) used for urgent surgical items. * **Prions:** Require higher parameters (134°C for 1-1.5 hours) for effective inactivation.

Question 75: Incineration is done for which of the following types of waste?

A. Liquid waste
B. Anatomical waste (Correct Answer)
C. Sharp waste
D. All of the above

Explanation: **Explanation:** **Incineration** is a high-temperature dry oxidation process that reduces organic and combustible waste to inorganic, incombustible ash. It is the preferred method for waste that requires complete destruction to ensure safety and aesthetics. **Why Anatomical Waste is Correct:** According to the **Bio-Medical Waste (BMW) Management Rules**, human anatomical waste (tissues, organs, body parts) and animal anatomical waste are categorized under **Yellow Bag** waste. Incineration is the gold standard for this category because it ensures the total destruction of pathogens and prevents the illegal reuse of body parts. It also significantly reduces the volume of the waste. **Why Other Options are Incorrect:** * **Liquid Waste:** This should be treated using chemical disinfection (e.g., 1% hypochlorite) followed by neutralization and discharge into drains. Incineration is not energy-efficient for liquids. * **Sharp Waste:** Sharps (needles, blades) are disposed of in **White (Translucent) puncture-proof containers**. They are treated by autoclaving or dry heat sterilization followed by shredding/mutilation. Incinerating sharps can lead to the release of toxic metal vapors and does not destroy the metal itself. **High-Yield Clinical Pearls for NEET-PG:** * **Temperature Requirements:** Incinerators have two chambers: the Primary chamber (800° ± 50°C) and the Secondary chamber (1050° ± 50°C). * **PVC Contraindication:** Never incinerate chlorinated plastics (PVC) like IV sets or blood bags, as this releases highly toxic **dioxins and furans**. * **Cytotoxic Drugs:** These must be incinerated at higher temperatures (>1200°C) in the yellow bag category. * **Ash Disposal:** The residue (incineration ash) is disposed of in a **Secured Landfill**.

Question 76: In a hot air oven, what is the required temperature for a holding period of one hour?

A. 100°C
B. 120°C
C. 140°C
D. 160°C (Correct Answer)

Explanation: **Explanation:** The **Hot Air Oven** is the most common method of **dry heat sterilization**. It works primarily through the oxidation of intracellular proteins, dehydration, and the toxic effects of elevated electrolyte concentrations within the microbial cell. **Why 160°C is correct:** Sterilization efficiency in a hot air oven is a function of both temperature and time (the "holding period"). For a holding period of **60 minutes (1 hour)**, the standard required temperature is **160°C**. This duration is necessary because dry heat has less penetrative power than moist heat, requiring higher temperatures and longer exposure times to ensure the destruction of highly resistant bacterial spores (e.g., *Clostridium tetani*). **Analysis of Incorrect Options:** * **100°C (Option A):** This is the boiling point of water. While it kills vegetative forms, it does not reliably kill spores and is considered disinfection, not sterilization. * **120°C (Option B):** This is close to the temperature used in an Autoclave (121°C), which utilizes moist heat under pressure. Dry heat at this temperature is insufficient for sterilization within an hour. * **140°C (Option C):** This temperature requires a longer holding period of at least **180 minutes (3 hours)** to achieve sterilization. **High-Yield Clinical Pearls for NEET-PG:** * **Sterilization Cycles:** 160°C for 1 hour, 170°C for 30 minutes, or 180°C for 15 minutes. * **Biological Indicator:** *Bacillus atrophaeus* (formerly *B. subtilis var. niger*). * **Uses:** Ideal for glassware (petri dishes, pipettes), forceps, scalpels, and non-aqueous substances like liquid paraffin, fats, and dusting powders. * **Contraindication:** Not suitable for surgical dressings, rubber items, or plastic, as they will char or melt.

Question 77: What is the main disinfecting agent of chlorine during water chlorination?

A. Hypochlorite ion
B. Chloride ion
C. Sodium chloride
D. Hypochlorous acid (Correct Answer)

Explanation: **Explanation:** When chlorine is added to water, it reacts to form two main compounds: **Hypochlorous acid (HOCl)** and **Hypochlorite ion (OCl⁻)**. Together, these are known as "Free Available Chlorine." **1. Why Hypochlorous Acid is the Correct Answer:** Hypochlorous acid (HOCl) is the **most effective** disinfecting agent. It is electrically neutral and possesses a low molecular weight, allowing it to easily penetrate the negatively charged cell walls of microorganisms. Once inside, it oxidizes essential enzymes and proteins, leading to cell death. It is approximately **70–80 times more effective** at killing bacteria than the hypochlorite ion. **2. Analysis of Incorrect Options:** * **Hypochlorite ion (OCl⁻):** While it has some germicidal activity, its negative charge causes it to be repelled by the bacterial cell wall, making it much slower and less efficient than HOCl. * **Chloride ion (Cl⁻):** This is a stable, non-oxidizing ion (like that found in common salt) and possesses no disinfecting properties. * **Sodium chloride (NaCl):** This is common table salt; it does not act as a disinfectant in the context of water chlorination. **3. High-Yield Clinical Pearls for NEET-PG:** * **pH Dependency:** The efficacy of chlorination is highly pH-dependent. At a lower pH (acidic), HOCl predominates; at a higher pH (alkaline), OCl⁻ predominates. Therefore, chlorination is most effective at a **pH of 7 or below**. * **Contact Time:** For effective disinfection, a minimum contact time of **30 to 60 minutes** is required. * **Free Residual Chlorine:** After the "chlorine demand" of the water is met, there should be a residual of **0.5 mg/L** of free chlorine to protect against subsequent post-treatment contamination. * **Cyclops:** Chlorination is ineffective against *Cyclops* (intermediate host of Guinea worm) and certain protozoal cysts (like *Amoeba*) at standard doses.

Question 78: Which of the following is an example of a high-level disinfectant?

A. Phenol
B. Glutaraldehyde (Correct Answer)
C. Alcohol
D. Povidone Iodine

Explanation: **Explanation:** Disinfectants are categorized based on their efficacy against microorganisms into High, Intermediate, and Low levels. **1. Why Glutaraldehyde is Correct:** **Glutaraldehyde** is a **High-Level Disinfectant (HLD)**. It works by alkylation of sulfhydryl, hydroxyl, carboxyl, and amino groups, which alters RNA, DNA, and protein synthesis. Crucially, HLDs are capable of destroying all microorganisms, including vegetative bacteria, mycobacteria, fungi, and viruses. While they can kill bacterial spores with prolonged exposure (making them "chemical sterilants"), in standard clinical practice, they are used as HLDs for heat-sensitive equipment. **2. Analysis of Incorrect Options:** * **Phenol (Option A):** This is a **Low to Intermediate-level disinfectant**. It acts by cell membrane disruption and protein precipitation. It is not effective against bacterial spores or all viruses. * **Alcohol (Option B):** 70% Ethyl or Isopropyl alcohol are **Intermediate-level disinfectants**. They denature proteins but lack sporicidal activity and are ineffective against non-enveloped viruses. * **Povidone Iodine (Option D):** An iodophor, which is an **Intermediate-level disinfectant/antiseptic**. It acts by iodinating proteins and oxidizing cell constituents but cannot reliably kill spores. **3. High-Yield NEET-PG Pearls:** * **Glutaraldehyde (Cidex):** Used for "cold sterilization" of endoscopes, cystoscopes, and bronchoscopes. Standard immersion time for HLD is 20 minutes (2% solution). * **Ortho-phthalaldehyde (OPA):** A newer HLD that is more stable and faster-acting than glutaraldehyde. * **Spaulding’s Classification:** Semicritical items (those touching mucous membranes but not entering sterile body cavities) require at least High-Level Disinfection.

Question 79: Tyndallisation is a type of?

A. Intermittent sterilization (Correct Answer)
B. Pasteurisation
C. Boiling
D. Autoclaving

Explanation: **Explanation:** **Tyndallisation** (also known as fractional sterilization) is a method of **intermittent sterilization** that uses moist heat at 100°C for 20 minutes on three successive days. The underlying principle is to kill both vegetative cells and highly resistant bacterial spores. On the first day, the heat kills vegetative bacteria but spores survive. During the subsequent incubation periods (at room temperature), these surviving spores germinate into vegetative forms, which are then destroyed by the heating process on the second and third days. This method is specifically used for media containing heat-sensitive ingredients like eggs, serum, or sugars that would be damaged by the high pressure of an autoclave. **Why other options are incorrect:** * **Pasteurisation:** This is a process of heating liquids (like milk) to specific temperatures (e.g., 63°C for 30 mins or 72°C for 15 secs) to kill pathogens without altering the flavor. It is a disinfection process, not sterilization, as it does not kill spores. * **Boiling:** Boiling at 100°C for 10–30 minutes kills most vegetative bacteria but is unreliable for killing spores; therefore, it is considered disinfection, not sterilization. * **Autoclaving:** This is sterilization by steam under pressure (121°C at 15 psi for 15 mins). It is a single-cycle process that kills all microorganisms, including spores, in one go. **High-Yield Clinical Pearls for NEET-PG:** * **Temperature:** Tyndallisation occurs at **100°C**, whereas Inspissation (used for LJ media) occurs at **80–85°C**. * **Application:** Used for media that cannot withstand autoclaving (e.g., sugar media). * **Key Concept:** The "intermittent" nature is the defining feature that allows for spore germination between heating cycles.

Question 80: Which of the following statements about Ethylene oxide is FALSE?

A. It has high penetration power.
B. It is highly inflammable.
C. Sterilization control used is Bacillus pumilus. (Correct Answer)
D. Ethylene oxide is irritant and carcinogenic.

Explanation: Ethylene oxide (EtO) is a potent alkylating agent used for the gaseous sterilization of heat-sensitive items. Understanding its properties and monitoring is high-yield for NEET-PG. ### **Why Option C is False** The correct biological indicator for Ethylene oxide sterilization is **_Bacillus atrophaeus_** (formerly known as *Bacillus subtilis var. niger*). * **_Bacillus pumilus_** is the biological indicator specifically used for **Ionizing Radiation** (Gamma rays). Using the wrong indicator would fail to validate the sterilization process for EtO. ### **Analysis of Other Options** * **Option A (High penetration power):** EtO is a gas with exceptional penetrating ability. It can pass through plastic films, cardboard, and complex lumens, making it ideal for pre-packaged medical devices like heart-lung machines and catheters. * **Option B (Highly inflammable):** In its pure form, EtO is highly explosive and flammable. To mitigate this risk, it is usually used in a diluted form (e.g., 10% EtO with 90% CO₂ or with hydrofluorocarbons). * **Option D (Irritant and Carcinogenic):** EtO is toxic. It is a known skin and mucosal irritant and is classified as a mutagen and carcinogen. Therefore, sterilized items must undergo a period of **aeration** to remove residual gas before patient use. ### **Clinical Pearls for NEET-PG** * **Mechanism of Action:** Alkylation of amino, carboxyl, and hydroxyl groups in bacterial proteins and nucleic acids. * **Best for:** Heart-lung machines, respirators, sutures, dental equipment, and disposable plastic syringes. * **Biological Indicators Summary:** * **Autoclave:** *Geobacillus stearothermophilus* * **Hot Air Oven:** *Bacillus atrophaeus* * **Ethylene Oxide:** *Bacillus atrophaeus* * **Ionizing Radiation:** *Bacillus pumilus* * **Plasma Sterilization:** *Geobacillus stearothermophilus*

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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