Sterilization and Disinfection — MCQs

Sterilization and Disinfection Indian Medical PG Practice Questions and MCQs

Question 151: All of the following are sporicidal agents EXCEPT:

A. Bleaching powder and ethylene oxide
B. Ethyl and isopropyl alcohol (Correct Answer)
C. Iodine and chlorine compounds
D. Hydrogen peroxide and bleaching powder

Explanation: **Explanation:** The core concept in sterilization is the distinction between **disinfectants** (which kill vegetative forms) and **sterilants** (which kill all microbial life, including highly resistant bacterial spores). **Why Option B is correct:** **Alcohols (Ethyl and Isopropyl)** are intermediate-level disinfectants. Their mechanism of action involves denaturing proteins and dissolving lipid membranes. While they are highly effective against vegetative bacteria, fungi, and enveloped viruses, they **lack sporicidal activity** because they cannot penetrate the thick, multi-layered protein coat of bacterial spores (like those of *Bacillus* and *Clostridium*). **Why other options are incorrect:** * **Ethylene Oxide (A):** A potent alkylating agent used for gaseous sterilization of heat-sensitive items. It is highly sporicidal. * **Halogens (C & D):** **Chlorine (Bleaching powder)** and **Iodine** are oxidizing agents. In appropriate concentrations and contact times, they are sporicidal. Chlorine is frequently used for disinfecting water and surfaces contaminated with spores. * **Hydrogen Peroxide (D):** High concentrations (stabilized 6–30%) act as high-level disinfectants/sterilants by producing free radicals that disrupt spore proteins and DNA. **NEET-PG High-Yield Pearls:** * **Alcohols:** Most effective at **70% concentration**; 100% alcohol is less effective as it causes rapid coagulation of surface proteins, preventing deeper penetration. * **Sporicidal Agents (The "Big Five"):** Glutaraldehyde (2%), Formaldehyde, Ethylene Oxide, Hydrogen Peroxide, and Peracetic acid. * **Prions:** Most resistant to sterilization. Standard autoclaving is insufficient; requires 1N NaOH for 1 hour followed by autoclaving at 134°C. * **Flash Sterilization:** 134°C for 3 minutes (used for urgent surgical instruments).

Question 152: Glutaraldehyde is used for sterilization of which of the following?

A. Endoscopes
B. Corrugated rubber anaesthetic tube
C. Plastic endotracheal tube
D. All of the above (Correct Answer)

Explanation: **Explanation:** Glutaraldehyde (Cidex) is a high-level disinfectant and chemical sterilant widely used in clinical settings. It acts by **alkylation of sulfhydryl, hydroxyl, carboxyl, and amino groups**, which effectively disrupts RNA, DNA, and protein synthesis in microorganisms. **Why "All of the above" is correct:** Glutaraldehyde is the agent of choice for "cold sterilization" of heat-sensitive medical equipment. * **Endoscopes (Option A):** It is the gold standard for disinfecting flexible fiberoptic endoscopes (e.g., gastroscopes, bronchoscopes) because it is non-corrosive to metals, rubber, and lenses. * **Anaesthetic and Respiratory Equipment (Options B & C):** Corrugated rubber tubes and plastic endotracheal tubes are thermolabile (heat-sensitive). Glutaraldehyde effectively sterilizes these materials without causing the melting or degradation that would occur in an autoclave. **Key Clinical Concepts:** * **Concentration:** It is typically used as a **2% buffered solution**. * **Exposure Time:** To achieve **disinfection** (killing vegetative bacteria, fungi, and viruses), an immersion time of **20 minutes** is required. To achieve **sterilization** (killing highly resistant spores), an immersion time of **10 hours** is necessary. * **Stability:** Once "activated" by adding an alkalinizing agent, the solution remains stable and effective for **14 days**. **High-Yield NEET-PG Pearls:** 1. **Ortho-phthalaldehyde (OPA):** A newer alternative to glutaraldehyde that does not require activation and is less irritating to the eyes and nasal passages. 2. **Cidex Test:** Used to monitor the minimum effective concentration (MEC) of the solution. 3. **Toxicity:** Glutaraldehyde fumes can cause asthma and dermatitis; instruments must be thoroughly rinsed with sterile water after immersion to prevent tissue irritation (e.g., chemical colitis from endoscopes).

Question 153: Irradiation can be used to sterilize all except:

A. Bone graft
B. Suture
C. Artificial tissue graft
D. Bronchoscope (Correct Answer)

Explanation: **Explanation:** The correct answer is **Bronchoscope** because it is a heat-sensitive, complex optical instrument that requires high-level disinfection or sterilization using chemical methods rather than irradiation. **1. Why Bronchoscope is the correct answer:** Bronchoscopes contain delicate components, including fiberoptic bundles, lenses, and rubber/plastic sheaths. **Ionizing radiation (Gamma rays)** can damage these materials, leading to clouding of the lenses or brittleness of the tubing. In clinical practice, bronchoscopes are typically sterilized using **Glutaraldehyde (2%)**, **Ortho-phthalaldehyde (OPA)**, or **Ethylene Oxide (EtO) gas**. **2. Why the other options are incorrect:** * **Bone grafts & Artificial tissue grafts:** Irradiation (Cold Sterilization) is the method of choice for biological tissues. It effectively penetrates dense materials to eliminate pathogens without the need for high heat, which would denature the proteins in the graft. * **Sutures:** Most disposable medical supplies, including catgut and synthetic sutures, are sterilized on a commercial scale using **Gamma radiation (Cobalt-60 source)**. This ensures they are sterile within their final packaging. **Clinical Pearls for NEET-PG:** * **Cold Sterilization:** This term refers to sterilization using **Ionizing Radiation** (Gamma rays, X-rays) because it produces negligible heat. It is ideal for heat-labile disposable items like syringes, catheters, and sutures. * **Standard Dose:** The recommended dose for gamma sterilization is **2.5 megarads (Mrad)**. * **Non-ionizing Radiation:** UV rays are used for disinfecting surfaces and air in OTs but have poor penetrating power compared to ionizing radiation. * **Bacillus pumilus:** This is the biological indicator used to test the efficacy of ionizing radiation.

Question 154: Which of the following is used for the sterilization of laparoscopic instruments?

A. 2% formalin
B. 2% glutaraldehyde (Correct Answer)
C. Autoclaving
D. Boiling

Explanation: **Explanation:** The correct answer is **2% glutaraldehyde (Cidex)**. Laparoscopic instruments are classified as "semi-critical" items because they come into contact with mucous membranes or non-intact skin. Since these instruments often contain delicate lenses and heat-sensitive components, they cannot withstand the high temperatures of autoclaving. **2% Glutaraldehyde** is the gold standard for "cold sterilization." It acts by alkylating the amino, carboxyl, and hydroxyl groups of proteins and nucleic acids. To achieve **high-level disinfection**, an immersion time of 20 minutes is required; however, to achieve true **sterilization** (killing of spores), the instruments must be immersed for **10 hours**. **Why other options are incorrect:** * **2% Formalin:** While it is a disinfectant, it is rarely used for instruments due to its slow action, pungent odor, and irritating fumes. It is primarily used for preserving tissues or as a gas for fumigating operation theaters. * **Autoclaving:** This is the method of choice for surgical steel instruments. However, the high temperature (121°C) and pressure can damage the fiber optics and cement in laparoscopic cameras and telescopes. * **Boiling:** This is a method of disinfection, not sterilization, as it does not reliably kill bacterial spores (e.g., *Clostridium* species). **High-Yield Clinical Pearls for NEET-PG:** * **Cidex Shelf Life:** Once "activated" by adding an alkalizing agent, the solution remains effective for **14 days**. * **Ortho-phthalaldehyde (OPA):** A newer alternative to glutaraldehyde that is more stable and does not require activation. * **Plasma Sterilization (H2O2):** Increasingly used for laparoscopes as a faster, non-toxic alternative to glutaraldehyde. * **Test for Efficacy:** The **Rideal-Walker coefficient** is used to compare a disinfectant's efficiency with phenol.

Question 155: Which of the following is an intermediate level disinfectant?

A. Glutaraldehyde
B. Quaternary ammonium compound
C. Hydrogen peroxide
D. Iodophores (Correct Answer)

Explanation: **Explanation:** Disinfectants are categorized into three levels based on their biocidal activity and the types of microorganisms they can kill. This classification is crucial for determining how medical equipment should be processed. **1. Why Iodophores are the Correct Answer:** **Iodophores** (e.g., Povidone-iodine) are classified as **Intermediate-level disinfectants**. They are effective against vegetative bacteria, most viruses (including HBV and HIV), and fungi. Crucially, they are **tuberculocidal**, which is the hallmark of intermediate-level activity. However, they do not reliably kill bacterial spores. **2. Analysis of Incorrect Options:** * **A. Glutaraldehyde:** This is a **High-level disinfectant (HLD)**. It is a "liquid sterilant" because, with sufficient contact time (e.g., 10 hours), it can destroy all microorganisms, including high numbers of bacterial spores. * **B. Quaternary Ammonium Compounds (QACs):** These are **Low-level disinfectants**. They are effective against most vegetative bacteria and enveloped viruses but are ineffective against mycobacteria (TB), non-enveloped viruses, and spores. * **C. Hydrogen Peroxide:** In high concentrations (6–25%), it acts as a **High-level disinfectant** or chemical sterilant. While low concentrations (3%) are used as antiseptics, it is generally categorized above the intermediate level in sterilization protocols. **3. High-Yield NEET-PG Clinical Pearls:** * **Spaulding’s Classification:** * **Critical items** (enter sterile tissue): Require Sterilization. * **Semi-critical items** (touch mucous membranes): Require High-level disinfection (e.g., Glutaraldehyde for endoscopes). * **Non-critical items** (touch intact skin): Require Low-level disinfection. * **The "TB Rule":** If a disinfectant kills *Mycobacterium tuberculosis*, it is at least an intermediate-level disinfectant. * **Common Intermediate Disinfectants:** Alcohols (70% Ethyl/Isopropyl), Phenolics, and Iodophores.

Question 156: What are the required conditions for effective autoclaving?

A. 121°C temperature for 20 minutes
B. 121°C temperature for 15 minutes (Correct Answer)
C. 100°C temperature for 60 minutes
D. 100°C temperature for 90 minutes

Explanation: **Explanation:** **1. Why Option B is Correct:** Autoclaving (Moist Heat Sterilization) operates on the principle of **saturated steam under pressure**. The standard holding period for effective sterilization is **121°C at 15 lbs (pounds) of pressure for 15 minutes**. At this temperature, the steam causes the irreversible denaturation and coagulation of structural proteins and enzymes within microorganisms. This process is highly effective because it destroys even the most resistant bacterial spores, such as *Bacillus stearothermophilus*. **2. Why Other Options are Incorrect:** * **Option A (121°C for 20 mins):** While 20 minutes will achieve sterilization, it is not the standard "minimum holding time" defined for routine autoclaving. Standard protocols prioritize 15 minutes to balance efficacy with the preservation of heat-sensitive materials. * **Options C & D (100°C):** Boiling at 100°C is a method of disinfection, not sterilization. It fails to kill highly resistant bacterial spores. Sterilization requires temperatures above 100°C, which can only be achieved by increasing the atmospheric pressure inside the autoclave chamber. **3. NEET-PG High-Yield Pearls:** * **Sterilization Indicator (Biological):** The most common indicator used to check the efficacy of an autoclave is the spores of ***Geobacillus stearothermophilus*** (formerly *Bacillus stearothermophilus*). * **Chemical Indicator:** **Bowie-Dick test** is used to detect air leaks and ensure steam penetration. **Browne’s tubes** change color (Red to Green) when the correct temperature and time are reached. * **Flash Sterilization:** A rapid method used in OTs for unwrapped instruments, typically at **134°C for 3 minutes**. * **Items Sterilized:** Culture media, surgical dressings, linen, and instruments. Note that **oils, powders, and sharp instruments** are better suited for Dry Heat (Hot Air Oven).

Question 157: A method of sterilization that involves exposure to 100°C for 20 minutes on 3 successive days is known as:

A. Autoclaving
B. Tyndallization (Correct Answer)
C. Pasteurization
D. Inspissation

Explanation: ### Explanation **Correct Option: B. Tyndallization** Tyndallization, also known as **intermittent or fractional sterilization**, involves heating materials at **100°C for 20 minutes on three successive days**. * **The Principle:** The first exposure kills all vegetative forms of bacteria. During the intervals (incubated at room temperature), any resistant **spores** germinate into vegetative forms. These newly germinated bacteria are then killed during the subsequent heating cycles on the second and third days. This method is used for media containing heat-sensitive ingredients like sugars or gelatin that might decompose at higher temperatures (e.g., in an autoclave). **Incorrect Options:** * **A. Autoclaving:** This is sterilization by steam under pressure, typically at **121°C for 15 minutes at 15 psi**. It is the most reliable method and kills both vegetative cells and spores in a single cycle. * **C. Pasteurization:** This is a method of disinfection (not sterilization) used primarily for milk. It involves lower temperatures (e.g., **63°C for 30 mins** or **72°C for 15 seconds**) and does not kill bacterial spores. * **D. Inspissation:** This involves heating at **80-85°C for 30 minutes for 3 successive days**. It is specifically used for media containing high amounts of protein (e.g., **Lowenstein-Jensen** or **Loeffler’s serum slope**) to solidify them without coagulation. **High-Yield NEET-PG Pearls:** * **Tyndallization vs. Inspissation:** Both are "fractional" methods, but Tyndallization uses 100°C (boiling), while Inspissation uses sub-boiling temperatures (80-85°C). * **Spores:** Tyndallization is specifically designed to tackle spores without using an autoclave. * **Commonly tested media:** Use Tyndallization for sugar media; use Inspissation for LJ media.

Question 158: Which of the following is least susceptible to disinfectants and antiseptics?

A. Spores
B. Protozoa
C. Prions (Correct Answer)
D. Fungi

Explanation: **Explanation:** The correct answer is **Prions**. The susceptibility of microorganisms to chemical disinfectants and sterilization follows a well-defined hierarchy. **Prions** are at the absolute top of this hierarchy, representing the most resistant infectious agents known. Unlike bacteria or viruses, prions are misfolded proteins devoid of nucleic acids. They are highly resistant to standard autoclaving, boiling, and common disinfectants (like alcohols or phenols) because they lack a lipid envelope or genetic material that these agents typically target. **Analysis of Options:** * **Spores (Option A):** While bacterial spores (e.g., *Bacillus anthracis*, *Clostridium*) are highly resistant to environmental stress and many disinfectants, they are still susceptible to high-level disinfectants (sporicides) and standard sterilization (autoclaving at 121°C). Prions remain more resistant than spores. * **Protozoa (Option B):** Protozoal cysts (like *Giardia*) are moderately resistant, but the vegetative forms are quite fragile and easily killed by standard disinfection. * **Fungi (Option D):** Most fungi and their asexual spores are susceptible to intermediate-level disinfectants. They are significantly less resistant than bacterial spores and prions. **NEET-PG High-Yield Pearls:** * **Hierarchy of Resistance (Highest to Lowest):** Prions > Bacterial Spores > Mycobacteria > Non-enveloped viruses > Fungi > Vegetative bacteria > Enveloped viruses (e.g., HIV, HBV). * **Prion Decontamination:** Standard autoclaving is insufficient. The recommended method is **autoclaving at 134°C for 18 minutes** or immersion in **1N Sodium Hydroxide (NaOH)** for 1 hour. * **Glutaraldehyde (2%):** A high-level disinfectant (Sterilant) effective against spores but **not** reliable against prions.

Question 159: High-level disinfectants are used for which of the following items?

A. Stethoscopes
B. Electronic thermometers
C. Bronchoscopes (Correct Answer)
D. Surgical instruments

Explanation: ### Explanation The classification of medical devices and their required level of decontamination is based on the **Spaulding Classification system**, which categorizes items into three groups based on the risk of infection. **1. Why Bronchoscopes (Option C) is correct:** Bronchoscopes are classified as **Semi-critical items**. These are devices that come into contact with mucous membranes or non-intact skin but do not penetrate sterile tissues. They require **High-Level Disinfection (HLD)**, which destroys all microorganisms except for high numbers of bacterial spores. Common HLD agents include 2% Glutaraldehyde (Cidex), Ortho-phthalaldehyde (OPA), and Peracetic acid. **2. Why the other options are incorrect:** * **Stethoscopes and Electronic thermometers (Options A & B):** These are **Non-critical items** as they only come into contact with intact skin. They require only **Low-level disinfection** (e.g., 70% isopropyl alcohol or quaternary ammonium compounds). * **Surgical instruments (Option D):** These are **Critical items** because they enter sterile tissue or the vascular system. These items must undergo **Sterilization** (complete destruction of all microbial life, including spores), typically via autoclaving. **High-Yield Clinical Pearls for NEET-PG:** * **Glutaraldehyde (Cidex):** The standard HLD for endoscopes. Requires 20 minutes for disinfection but 10 hours for "cold sterilization." Once activated, the solution is stable for 14 days. * **Prions:** Standard HLD and routine autoclaving (121°C) are insufficient for prions. They require 134°C for 18 minutes or immersion in 1N NaOH. * **Spore test:** The biological indicator for autoclaving is *Geobacillus stearothermophilus*.

Question 160: Which of the following is used as a biological indicator during plasma gas sterilization?

A. Bacillus subtilis
B. Geobacillus stearothermophilus (Correct Answer)
C. Clostridium tetani
D. Clostridium perfringens

Explanation: ### Explanation **Correct Answer: B. Geobacillus stearothermophilus** Biological indicators (BIs) are the most reliable method for monitoring the sterilization process because they utilize highly resistant bacterial spores to directly measure the "microbial kill." **Geobacillus stearothermophilus** is the standard biological indicator for **Plasma Gas (Hydrogen Peroxide) sterilization**. It is chosen because its spores are highly resistant to oxidative stress. Once the sterilization cycle is complete, the indicator is incubated; a lack of growth (no color change/fluorescence) confirms that the process was successful in achieving surgical sterility. **Analysis of Incorrect Options:** * **A. Bacillus subtilis (now B. atrophaeus):** This is the biological indicator for **Ethylene Oxide (ETO)** and **Dry Heat sterilization** (Hot Air Oven). It is not used for plasma sterilization. * **C & D. Clostridium tetani and Clostridium perfringens:** While these are spore-forming organisms, they are obligate anaerobes and human pathogens. They are never used as indicators in clinical settings due to safety risks and specific growth requirements. --- ### High-Yield Clinical Pearls for NEET-PG: * **Autoclave (Moist Heat):** Uses *Geobacillus stearothermophilus*. * **Plasma Sterilization (H2O2):** Uses *Geobacillus stearothermophilus*. * **Hot Air Oven (Dry Heat):** Uses *Bacillus atrophaeus* (formerly *B. subtilis* var. *niger*). * **Ethylene Oxide (ETO):** Uses *Bacillus atrophaeus*. * **Ionizing Radiation (Gamma rays):** Uses *Bacillus pumilus*. * **Filtration:** Uses *Brevundimonas diminuta*. **Note:** Plasma sterilization is the preferred method for heat-sensitive equipment like endoscopes and robotic instruments because it is rapid and leaves no toxic residue.

Practice by Chapter

Physical Methods of Sterilization

Practice Questions

Chemical Methods of Sterilization

Practice Questions

Disinfectants and Antiseptics

Practice Questions

Sterilization Monitoring

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Hospital Sterilization Protocols

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Surgical Instruments Sterilization

Practice Questions

Endoscope Reprocessing

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Biological Indicators

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Factors Affecting Sterilization

Practice Questions

Quality Control in Sterilization

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Biofilm and Its Implications

Practice Questions

Regulatory Aspects of Sterilization

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