Sterilization and Disinfection — MCQs

Sterilization and Disinfection Indian Medical PG Practice Questions and MCQs

Question 41: Autoclave is used for sterilization of which of the following materials?

A. Wooden material
B. Metallic instrument (Correct Answer)
C. Plastic
D. Glasswares

Explanation: **Explanation:** The **Autoclave** operates on the principle of **moist heat sterilization** using saturated steam under pressure (standard conditions: 121°C at 15 lbs pressure for 15–20 minutes). This process kills microorganisms and highly resistant spores by **denaturation and coagulation of structural proteins and enzymes.** **Why Metallic Instruments are the Correct Choice:** Stainless steel metallic instruments are heat-stable and can withstand high pressure and moisture without losing their structural integrity. Moist heat has better penetration than dry heat, making it the gold standard for surgical instruments, linen, and culture media. **Analysis of Incorrect Options:** * **Wooden material:** Wood is porous and can trap moisture, leading to warping, splintering, or incomplete sterilization. It is generally not autoclaved. * **Plastic:** Most standard plastics (like polyethylene) have low melting points and will melt or deform under the high temperatures of an autoclave. Only specific "autoclavable" plastics (like polypropylene) can be processed this way. * **Glasswares:** While some heat-resistant glass (Pyrex) can be autoclaved, the **preferred** method for empty glass syringes, petri dishes, and flasks is the **Hot Air Oven (Dry Heat)** to prevent moisture condensation and "clouding" of the glass. **High-Yield Clinical Pearls for NEET-PG:** * **Sterilization Control:** The biological indicator used to check autoclave efficacy is **_Geobacillus stearothermophilus_** (formerly *Bacillus stearothermophilus*). * **Chemical Indicator:** **Browne’s tubes** (color change) or **Bowie-Dick test** (for vacuum efficiency). * **Flash Sterilization:** 134°C for 3 minutes (used for urgent surgical needs). * **Prions:** Require higher settings (134°C for 1–1.5 hours) for effective inactivation.

Question 42: Prions are inactivated by which of the following methods?

A. Autoclave and NaOH (Correct Answer)
B. Ethylene dioxide
C. Gamma radiation
D. Autoclave

Explanation: **Explanation:** Prions are highly resistant, misfolded proteins that lack nucleic acids, making them the most difficult infectious agents to inactivate. Standard sterilization methods that typically kill bacteria, viruses, and spores are often ineffective against prions. **1. Why "Autoclave and NaOH" is correct:** To ensure complete inactivation of prions (e.g., in cases of Creutzfeldt-Jakob Disease), a combination of physical and chemical methods is required. The recommended protocol involves immersing instruments in **1N Sodium Hydroxide (NaOH)** for one hour, followed by gravity displacement **autoclaving at 121°C for 30–60 minutes**. The NaOH acts by denaturing the protein structure, while the extended autoclave cycle ensures thermal degradation of the remaining protein aggregates. **2. Why other options are incorrect:** * **Ethylene Oxide (ETO):** This is a low-temperature gaseous sterilant used for heat-sensitive items. It is ineffective against prions as it primarily targets nucleic acids and functional groups, which prions lack. * **Gamma Radiation:** Prions are remarkably resistant to ionizing radiation because their "target size" (the protein molecule) is much smaller than the genome of a virus or bacterium. * **Autoclave (Standard):** A standard autoclave cycle (121°C for 15 mins) is insufficient. Prions can survive standard pressurized steam unless the temperature is increased (to 134°C) or the duration is significantly extended and combined with chemical treatment. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Resistance:** Prions > Bacterial Spores (*B. subtilis*) > Mycobacteria > Non-enveloped viruses > Fungi > Vegetative bacteria > Enveloped viruses (HIV/HBV). * **Specific Protocol:** The WHO-recommended method is **1N NaOH + Autoclaving at 121°C for 60 minutes**. * **Alternative:** If NaOH cannot be used, autoclaving at **134°C for 18 minutes** (Pre-vacuum) is an accepted alternative. * **Disinfectants to Avoid:** Alcohol, formaldehyde, and glutaraldehyde are ineffective and may actually "fix" the prion proteins to the instruments, making them harder to remove.

Question 43: Which of the following is NOT a vapor-phase disinfectant?

A. Ethylene oxide
B. Thiomersal (Correct Answer)
C. Formaldehyde gas
D. Betapropiolactone

Explanation: ### Explanation **Concept:** Sterilization and disinfection agents are classified based on their physical state during application. **Vapor-phase (gaseous) disinfectants** are used to sterilize heat-sensitive items or large spaces (fumigation) because they can penetrate porous surfaces and reach inaccessible areas. **1. Why Thiomersal is the correct answer:** Thiomersal (Thimerosal) is an **organomercurial compound** used as a **liquid preservative**, not a gas or vapor. It is primarily used in multi-dose vaccine vials and ophthalmic solutions to prevent bacterial and fungal growth. It acts by inhibiting bacterial enzymes through the binding of mercury to sulfhydryl groups. **2. Analysis of Incorrect Options (Vapor-phase agents):** * **Ethylene Oxide (EtO):** A highly penetrating gas used for heat-sensitive items like plastic syringes, heart-lung machines, and catheters. It acts via alkylation. * **Formaldehyde Gas:** Used for the fumigation of operation theaters and biological safety cabinets. It is generated by heating paraformaldehyde or adding potassium permanganate to formalin. * **Betapropiolactone (BPL):** A condensation product of ketone and formaldehyde. It is a potent sporicidal agent used in the gaseous state for inactivating viruses in vaccine preparation and for fumigation. It is preferred over formaldehyde for some uses as it is more efficient and non-carcinogenic (though it is a suspected carcinogen, it is less persistent). **High-Yield Clinical Pearls for NEET-PG:** * **Sterilization of Choice:** For disposable plastics (syringes) = **Ethylene Oxide**. * **Fumigation Agent of Choice:** Traditionally **Formaldehyde**; however, **Hydrogen Peroxide vapor** is increasingly preferred due to its non-toxic residues (breaks down into water and oxygen). * **BPL vs. Formaldehyde:** BPL is more rapid and requires lower concentrations but has poor penetrating power compared to EtO. * **Thiomersal Fact:** It contains ethylmercury and has been largely phased out of childhood vaccines as a precautionary measure, though no evidence of toxicity at preservative levels exists.

Question 44: What is the best method for sterilizing all-glass syringes?

A. Boiling
B. Autoclave
C. Hot air oven (Correct Answer)
D. Formaldehyde

Explanation: ### Explanation **Correct Answer: C. Hot Air Oven** The **Hot Air Oven (Dry Heat Sterilization)** is the preferred method for all-glass syringes because glass is a poor conductor of heat and requires high temperatures for a sustained period to ensure sterility. Dry heat is non-corrosive for glass and prevents the condensation of moisture, which can occur with steam methods. The standard cycle is **160°C for 1 hour**. It effectively kills spores and eliminates pyrogens, which is critical for equipment used in parenteral injections. **Why other options are incorrect:** * **Boiling (A):** Boiling is a method of disinfection, not sterilization. It does not reliably kill bacterial spores (e.g., *Clostridium tetani*) and can lead to the accumulation of scale/mineral deposits on the glass. * **Autoclave (B):** While autoclaving (Moist Heat) is the "gold standard" for most surgical instruments, it is less ideal for all-glass syringes. Moisture can get trapped between the plunger and the barrel, potentially causing the glass to crack or jam due to differential expansion. * **Formaldehyde (D):** This is a high-level disinfectant/gas sterilant used for heat-sensitive items (like endoscopes). It is unnecessary for glass, leaves toxic residues, and requires long neutralization periods. **High-Yield Clinical Pearls for NEET-PG:** * **Dry Heat** is also the method of choice for **glassware** (petri dishes, pipettes), **powders**, and **oily substances** (liquid paraffin, fats). * **Sterilization Control:** The biological indicator for a Hot Air Oven is ***Bacillus atrophaeus*** (formerly *B. subtilis* var. *niger*). * **Contraindication:** Never use a Hot Air Oven for surgical dressings, rubber items, or plastics, as they will burn or melt.

Question 45: Moist heat kills all of the following microorganisms except:

A. Brucella
B. Mycobacteria
C. Salmonella
D. Coxiella burnetti (Correct Answer)

Explanation: **Explanation:** The question refers to the efficacy of **Pasteurization**, a specific method of moist heat sterilization. Pasteurization is designed to kill non-spore-forming pathogenic bacteria in milk without altering its nutritional value. **1. Why Coxiella burnetii is the correct answer:** *Coxiella burnetii*, the causative agent of Q fever, is the **most heat-resistant non-spore-forming pathogen** found in milk. While standard pasteurization (Holder method: 63°C for 30 mins; HTST method: 72°C for 15-20 seconds) is designed to eliminate most pathogens, *Coxiella* can survive if the temperature or duration is slightly inadequate. It is used as the **indicator organism** for the efficiency of pasteurization; if *Coxiella* is killed, all other vegetative pathogens are assumed to be destroyed. **2. Analysis of incorrect options:** * **A. Brucella:** It is highly sensitive to heat and is effectively eliminated by standard pasteurization. Milk-borne transmission of Brucellosis is prevented by this process. * **B. Mycobacteria:** *Mycobacterium bovis* and *M. tuberculosis* were historically the primary targets of pasteurization. They are less heat-resistant than *Coxiella*. * **C. Salmonella:** These are common vegetative food-borne pathogens that are easily killed by moist heat at temperatures well below 60°C. **3. NEET-PG High-Yield Pearls:** * **Indicator of Pasteurization:** *Coxiella burnetii* (biological indicator); **Phosphatase Test** (biochemical indicator). * **Moist Heat (below 100°C):** Includes Pasteurization and Inspissation (used for Löwenstein-Jensen media). * **Moist Heat (at 100°C):** Boiling and Tyndallization (intermittent sterilization). * **Moist Heat (above 100°C):** Autoclaving (121°C for 15 mins at 15 psi), which kills even highly resistant spores (Indicator: *Geobacillus stearothermophilus*).

Question 46: Which of the following agents is effective in disinfecting spores?

A. Ethylene oxide
B. Beta propiolactone
C. Formaldehyde (Correct Answer)
D. Hexachlorophene

Explanation: **Explanation:** The core concept in sterilization is the distinction between **disinfectants** (which reduce microbial load) and **sterilants** (which eliminate all forms of microbial life, including highly resistant bacterial spores). **Why Formaldehyde is correct:** Formaldehyde is a high-level disinfectant and a potent alkylating agent. It acts by alkylating amino, carboxyl, and hydroxyl groups in nucleic acids and proteins. In its gaseous form or as a 10% buffered solution (formalin), it is **sporicidal**. It is traditionally used for disinfecting OT surfaces, fumigation, and preserving anatomical specimens. **Analysis of Incorrect Options:** * **Ethylene oxide (ETO):** While ETO is an excellent sporicidal agent, it is classified as a **gas sterilant** used for heat-sensitive items (like heart-lung machines or plastic syringes) rather than a routine liquid disinfectant. * **Beta-propiolactone (BPL):** BPL is also a sporicidal sterilant. However, it is primarily used for sterilizing biological products like vaccines and sera. It is not used as a general disinfectant due to its carcinogenic properties and poor penetrating power. * **Hexachlorophene:** This is a chlorinated bisphenol, which is a **low-level disinfectant**. It is bacteriostatic (mainly against Gram-positive bacteria) and has **no activity against spores**, viruses, or mycobacteria. **High-Yield Clinical Pearls for NEET-PG:** * **Glutaraldehyde (2%):** Known as "Cidex," it is the agent of choice for "cold sterilization" of endoscopes (requires 10 hours for sporicidal action). * **Plasma Sterilization:** Uses Hydrogen Peroxide vapors; it is the modern replacement for ETO for heat-sensitive equipment. * **Prions:** These are the most resistant to sterilization. They require autoclaving at 134°C for 1-1.5 hours or treatment with 1N NaOH.

Question 47: Ethyl alcohol (Ethanol) is more effective as a disinfectant when mixed with water in what concentration range?

A. 10 to 20 percent
B. 20 to 30 percent
C. 40 to 50 percent
D. 60 to 70 percent (Correct Answer)

Explanation: **Explanation:** The efficacy of ethyl alcohol as a disinfectant depends on its ability to denature proteins and dissolve lipid membranes. The correct answer is **60 to 70 percent** because the presence of water is essential for the denaturation process. 1. **Mechanism of Action:** Water acts as a catalyst in the denaturation of proteins. Pure alcohol (100%) causes rapid dehydration of the bacterial cell wall, which can lead to the formation of a protective crust. This prevents the alcohol from penetrating deeper into the cell. In contrast, a 60-70% aqueous solution slows evaporation and facilitates the entry of alcohol into the cytoplasm, ensuring lethal damage to internal proteins and enzymes. 2. **Incorrect Options:** * **10% to 50% (Options A, B, C):** These concentrations are too dilute. While they contain sufficient water, the alcohol content is insufficient to effectively disrupt the lipid bilayer or denature proteins within a clinically relevant timeframe. 3. **High-Yield Clinical Pearls for NEET-PG:** * **Spectrum:** Alcohols are bactericidal, tuberculocidal, and fungicidal. They are effective against enveloped viruses (e.g., HIV, HBV, Influenza) but are **not sporicidal**. * **Isopropyl Alcohol:** Often preferred over ethanol as it is slightly more potent and less volatile. * **Hand Rubs:** The WHO-recommended hand rub formulation typically uses 80% ethanol or 75% isopropyl alcohol to ensure efficacy in clinical settings. * **Limitation:** Alcohols are inactivated by organic matter (pus, blood) and should only be used on clean surfaces or skin.

Question 48: Sterrad is a gas plasma sterilizer used in operation theaters for disinfecting OT articles. Which of the following is the active agent used in it?

A. H2O2 (Correct Answer)
B. N2O
C. Ozone
D. ETO gas

Explanation: **Explanation:** **Sterrad** is a low-temperature sterilization system that utilizes **Hydrogen Peroxide (H₂O₂) gas plasma** technology. It is increasingly preferred in modern operation theaters for heat-sensitive equipment. **1. Why H₂O₂ is Correct:** The process involves two phases. First, a concentrated H₂O₂ solution is vaporized into a chamber containing the instruments. Second, an electromagnetic field is applied to excite the gas molecules into a **plasma state**. This generates **free radicals** (hydroxyl and hydroperoxyl radicals) that disrupt the metabolism and cell walls of microorganisms, including highly resistant bacterial spores. It is preferred because it is rapid (30–75 mins) and leaves no toxic residues, as the by-products are simply water and oxygen. **2. Why Other Options are Incorrect:** * **B. N₂O (Nitrous Oxide):** This is an anesthetic gas ("laughing gas") and has no significant antimicrobial or sterilization properties. * **C. Ozone:** While ozone can be used for sterilization, it is not the active agent in the Sterrad system. Ozone sterilization is a separate process used primarily for water treatment or specific industrial applications. * **D. ETO (Ethylene Oxide):** ETO is a traditional low-temperature gaseous sterilant. However, it is highly toxic, carcinogenic, and requires long aeration times (up to 12 hours) to remove residues, unlike the rapid, non-toxic Sterrad process. **High-Yield Clinical Pearls for NEET-PG:** * **Indications:** Best for heat-sensitive and moisture-sensitive items like **endoscopes, fiber-optic cables, and power tools.** * **Contraindications:** Cannot be used for **cellulose-based materials** (paper, linen, cotton) or liquids, as they absorb the H₂O₂ and abort the cycle. * **Monitoring:** The biological indicator used for H₂O₂ gas plasma is ***Geobacillus stearothermophilus***.

Question 49: How is sputum disinfected?

A. Filtration
B. Hypochlorite
C. Incineration (Correct Answer)
D. Ionizing radiation

Explanation: **Explanation:** The primary objective in disinfecting sputum, particularly in a clinical setting, is the complete destruction of highly resistant pathogens like *Mycobacterium tuberculosis*. **1. Why Incineration is Correct:** Incineration is the method of choice for disposing of infectious biological waste, including sputum, gauze, and anatomical waste. It involves high-temperature combustion (800°C–1100°C) that reduces organic matter to sterile ash. This ensures the total destruction of all microorganisms, including bacterial spores and acid-fast bacilli (AFB), which are often protected by the thick, mucoid matrix of sputum. **2. Why Other Options are Incorrect:** * **Filtration:** This is a physical method used to remove microbes from heat-sensitive liquids (e.g., sera, sugars) or air (HEPA filters). It is physically impossible to filter thick, viscous sputum. * **Hypochlorite:** While 1%–5% Sodium Hypochlorite is a potent disinfectant used for surface spills or blood, it is less effective for bulk sputum disposal because organic matter (mucus) neutralizes its activity, preventing penetration to the core of the sample. * **Ionizing Radiation:** Also known as "Cold Sterilization" (e.g., Gamma rays), this is used for heat-sensitive commercial items like disposable syringes, catheters, and sutures. It is not a practical or cost-effective method for routine bedside waste disposal. **Clinical Pearls for NEET-PG:** * **Burning vs. Incineration:** If incineration is unavailable, burning is the next best alternative for sputum cups. * **Chemical Alternative:** If sputum must be disinfected chemically before disposal, **5% Cresol** or **10% Bleach** for at least 1 hour is recommended. * **High-Yield Fact:** For laboratory-based decontamination of sputum (before culture), **4% NaOH (Petroff’s Method)** is used to kill contaminating flora while keeping *M. tuberculosis* viable.

Question 50: Pasteurization is a type of disinfection?

A. Precurrent (Correct Answer)
B. Concurrent
C. Preconcurrent
D. Terminal

Explanation: ### Explanation **1. Why "Precurrent" is Correct:** Disinfection is categorized based on the timing of the procedure relative to the presence of the source of infection. **Precurrent disinfection** (also known as prophylactic disinfection) refers to measures taken **before** the occurrence of an infection to prevent its onset. Pasteurization of milk is a classic example of precurrent disinfection. It involves heating milk to specific temperatures (e.g., 63°C for 30 mins or 72°C for 15 seconds) to kill pathogens like *Mycobacterium bovis*, *Brucella*, and *Salmonella* **before** they can reach the consumer and cause disease. Other examples include chlorination of water and handwashing. **2. Analysis of Incorrect Options:** * **Concurrent Disinfection:** This refers to the immediate disinfection of infectious discharges (sputum, feces, urine) and soiled articles **during** the course of an illness while the patient is still a source of infection. * **Terminal Disinfection:** This is the application of disinfectants **after** the patient has been discharged, transferred, or has died (e.g., fumigating a room after a TB patient leaves). * **Preconcurrent:** This is not a standard medical term in the classification of disinfection; it is a distractor. **3. High-Yield Clinical Pearls for NEET-PG:** * **Pasteurization is NOT sterilization:** It kills most pathogenic bacteria but does not eliminate bacterial spores or thermoduric organisms. * **Phosphatase Test:** This is the standard test to check the efficacy of pasteurization. If the enzyme phosphatase is destroyed, pasteurization is considered successful. * **Coxiella burnetii:** This is the most heat-resistant non-spore-forming pathogen found in milk; pasteurization parameters are specifically designed to ensure its destruction. * **Method Comparison:** * *Holder Method:* 63°C for 30 mins. * *Flash Method (HTST):* 72°C for 15 seconds.

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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