Sterilization and Disinfection — MCQs

Sterilization and Disinfection Indian Medical PG Practice Questions and MCQs

Question 161: How is a proctoscope typically sterilized?

A. Autoclave
B. Glutaraldehyde (Correct Answer)
C. Formalin
D. Soap

Explanation: **Explanation:** The sterilization of medical instruments is categorized by the **Spaulding Classification** based on the risk of infection. A proctoscope is classified as a **semi-critical item** because it comes into contact with mucous membranes or non-intact skin but does not penetrate sterile tissues. **Why Glutaraldehyde is correct:** Glutaraldehyde (2% solution, commonly known as Cidex) is the standard high-level disinfectant (HLD) for semi-critical instruments like endoscopes, proctoscopes, and cystoscopes. It is preferred because it is non-corrosive to metal and does not damage lenses or rubber components. To achieve HLD, an immersion time of 20 minutes is required, while 10 hours are needed for total sterilization (sporicidal action). **Analysis of incorrect options:** * **Autoclave:** While steam under pressure is the gold standard for sterilization, it is generally avoided for older proctoscopes or those with delicate light carriers/lenses due to the risk of heat-induced damage and corrosion. * **Formalin:** Formaldehyde gas or solution is pungent, irritating to the skin and mucosa, and potentially carcinogenic. It is rarely used for routine instrument disinfection today. * **Soap:** Soap and water are used only for preliminary cleaning to remove organic debris (bioburden) but do not provide the disinfection required for medical procedures. **High-Yield Clinical Pearls for NEET-PG:** * **Cidex Activation:** 2% Glutaraldehyde must be "activated" by adding an alkalizing agent (sodium bicarbonate) to become effective. Once activated, the solution is stable for **14 days**. * **Ortho-phthalaldehyde (OPA):** A newer alternative to glutaraldehyde that is faster-acting and does not require activation. * **Sterilization vs. Disinfection:** Remember, semi-critical items require High-Level Disinfection (HLD), whereas critical items (surgical tools) require absolute sterilization (Autoclave).

Question 162: Phenol coefficient indicates?

A. Efficiency of a disinfectant (Correct Answer)
B. Dilution of a disinfectant
C. Quantity of a disinfectant
D. Purity of a disinfectant

Explanation: ### Explanation **1. Why Option A is Correct:** The **Phenol Coefficient** (also known as the Rideal-Walker or Chick-Martin test) is a standardized measure used to determine the **germicidal efficiency** of a disinfectant. It compares the killing efficacy of a specific test disinfectant against a standard concentration of Phenol under identical conditions. * **The Formula:** Phenol Coefficient = (Highest dilution of test disinfectant killing the organism in 10 mins but not 5 mins) / (Highest dilution of phenol showing the same result). * A coefficient **>1** indicates the disinfectant is more effective than phenol, while **<1** indicates it is less effective. **2. Why Other Options are Incorrect:** * **Option B (Dilution):** While the test involves serial dilutions to find the endpoint, the coefficient itself is a ratio of performance, not a measure of the dilution required for clinical use. * **Option C (Quantity):** The test measures potency and quality of action, not the physical volume or mass of the agent. * **Option D (Purity):** Purity refers to the chemical composition and absence of contaminants, which is determined by chromatography or spectroscopy, not by its ability to kill bacteria. **3. High-Yield Clinical Pearls for NEET-PG:** * **Test Organisms:** The standard bacteria used for determining the Phenol Coefficient are *Salmonella typhi*, *Staphylococcus aureus*, and *Pseudomonas aeruginosa*. * **Rideal-Walker Test:** Uses water as the diluent; its main drawback is that it doesn't account for the presence of organic matter. * **Chick-Martin Test:** A modification that adds organic matter (like dried yeast or feces) to better simulate real-world clinical conditions. * **Phenol (Carbolic Acid):** It is the "gold standard" disinfectant against which others are compared, though it is now less commonly used clinically due to its toxicity and corrosive nature.

Question 163: Bacterial spores are best destroyed by?

A. UV rays
B. Autoclaving at 121°C for 20 minutes (Correct Answer)
C. Hot air oven
D. Infrared rays

Explanation: **Explanation:** **Why Autoclaving is the Correct Answer:** Autoclaving (Moist Heat Sterilization) is the most reliable method for destroying bacterial spores. It operates on the principle of **steam under pressure**, which allows water to reach temperatures above its boiling point. At **121°C for 20 minutes (at 15 psi)**, the moist heat causes **irreversible coagulation and denaturation of structural proteins and enzymes** within the spore. Moist heat has greater penetrating power than dry heat, making it the "gold standard" for sterilizing surgical instruments and culture media. **Analysis of Incorrect Options:** * **UV Rays:** These are a form of non-ionizing radiation used for surface disinfection and air sterilization. They have poor penetrating power and are ineffective against highly resistant spores. * **Hot Air Oven:** This uses dry heat (typically 160°C for 2 hours). While it can kill spores, it requires much higher temperatures and longer durations than autoclaving because dry heat kills by oxidation, which is less efficient than protein coagulation. * **Infrared Rays:** These are used primarily for rapid sterilization of pre-packed items (like syringes) in industrial settings via thermal effects, but they are not the standard clinical method for destroying spores compared to autoclaving. **High-Yield Clinical Pearls for NEET-PG:** * **Biological Indicator:** The sterilization efficacy of an autoclave is tested using spores of ***Geobacillus stearothermophilus*** (formerly *Bacillus stearothermophilus*). * **Dry Heat Indicator:** For Hot Air Ovens, the biological indicator is ***Bacillus atrophaeus*** (formerly *B. subtilis* var. *niger*). * **Flash Sterilization:** A variant of autoclaving performed at **134°C for 3 minutes**, used for urgent surgical requirements. * **Prion Sterilization:** Prions are highly resistant; autoclaving at **134°C for 1 hour** or using Sodium Hypochlorite is recommended.

Question 164: The Rideal-Walker test is used to determine the efficiency of which of the following?

A. Disinfectants (Correct Answer)
B. Moist heat sterilization
C. Antibiotics
D. Dry heat sterilization

Explanation: ### Explanation **Correct Option: A. Disinfectants** The **Rideal-Walker (RW) test** is a standard laboratory method used to evaluate the efficiency of water-soluble disinfectants, specifically those derived from phenol. It determines the **Phenol Coefficient**, which is a numerical value representing the germicidal power of a disinfectant compared to pure phenol. The test involves exposing a standard suspension of bacteria (usually *Salmonella typhi*) to varying dilutions of the disinfectant and phenol. If a disinfectant has an RW coefficient of 10, it means it is 10 times more effective than phenol under the test conditions. **Why other options are incorrect:** * **B & D (Moist and Dry Heat):** Sterilization by heat is monitored using physical (thermocouples), chemical (Browne’s tubes), or biological indicators (e.g., *Geobacillus stearothermophilus* for moist heat and *Bacillus atrophaeus* for dry heat). RW test is strictly for chemical agents. * **C (Antibiotics):** The efficacy of antibiotics is measured using **Antimicrobial Susceptibility Testing (AST)** methods, such as the Kirby-Bauer disk diffusion method or Minimum Inhibitory Concentration (MIC) broth dilution. **High-Yield Clinical Pearls for NEET-PG:** * **Chick-Martin Test:** A modification of the RW test that uses organic matter (yeast or feces) to simulate real-world conditions, making it more practical. * **In-use Test (Maurer’s Test):** Used to check the efficacy of disinfectants already in use in hospital wards/theaters to detect contamination or loss of potency. * **Standard Organisms:** *Salmonella typhi*, *Staphylococcus aureus*, and *Pseudomonas aeruginosa* are the standard strains used for testing disinfectants. * **Limitations:** The RW test does not account for the presence of organic matter and is not suitable for disinfectants that are not phenol-like.

Question 165: Plasma sterilization accuracy is assessed by using which of the following microorganisms?

A. Bacillus subtilis
B. Bacillus stearothermophilus (Correct Answer)
C. Staphylococcus aureus
D. Clostridium tetani

Explanation: **Explanation:** **Plasma Sterilization (Hydrogen Peroxide Gas Plasma)** is a low-temperature sterilization method used for heat-sensitive equipment like endoscopes and microsurgical instruments. It works by creating a reactive cloud of free radicals that disrupt the cell walls and DNA of microorganisms. **Why the correct answer is right:** The efficacy of any sterilization process is validated using a **Biological Indicator (BI)**, which consists of the most resistant microbial spores for that specific method. For Plasma Sterilization, the standard biological indicator is **_Bacillus stearothermophilus_** (specifically the *ATCC 7953* or *12980* strains). If the process can kill these highly resistant thermophilic spores, it is assumed that all other pathogens have been eliminated. **Analysis of Incorrect Options:** * **A. *Bacillus subtilis* (now *B. atrophaeus*):** This is the biological indicator used for **Dry Heat sterilization** (Hot Air Oven) and **Ethylene Oxide (ETO)** sterilization. * **C. *Staphylococcus aureus*:** This is a vegetative bacterium used in the **Rideal-Walker coefficient** and **Chick-Martin tests** to evaluate the efficacy of liquid disinfectants (phenols), but it is not resistant enough to serve as a sterilization indicator. * **D. *Clostridium tetani*:** While it forms spores, it is not used as a standardized indicator for sterilization monitoring in clinical settings. **High-Yield Clinical Pearls for NEET-PG:** * **Autoclave (Moist Heat):** Also uses _Bacillus stearothermophilus_ as a biological indicator. * **Ionizing Radiation (Gamma rays):** Uses _Bacillus pumilus_. * **Filtration:** Uses _Brevundimonas diminuta_. * **Plasma Sterilization Advantage:** It is non-toxic (byproducts are only water and oxygen) and has a very short cycle time compared to ETO.

Question 166: Which of the following is NOT a sterilizing/disinfecting agent?

A. Dry heat
B. Ethylene oxide
C. Ether (Correct Answer)
D. Alcohol

Explanation: **Explanation:** The correct answer is **Ether**. In microbiology, sterilization refers to the absolute destruction of all forms of microbial life, including spores, while disinfection refers to the reduction of pathogenic organisms to a safe level. **Why Ether is the correct answer:** Ether is primarily used as a volatile general anesthetic or a solvent. While it can dissolve lipid membranes, it is **not** classified as a standard disinfectant or sterilizing agent because it is highly flammable, explosive, and lacks sufficient germicidal efficacy for clinical sterilization protocols. **Analysis of other options:** * **Dry Heat:** A physical method of sterilization (e.g., Hot Air Oven). It acts by protein denaturation, oxidative damage, and toxic effects of elevated electrolyte levels. It is the method of choice for glassware and powders. * **Ethylene Oxide (EtO):** A potent gaseous chemical sterilant. It acts via **alkylation** of DNA and proteins. It is the "gold standard" for sterilizing heat-sensitive items like plastics, heart-lung machines, and catheters. * **Alcohol:** A common disinfectant (e.g., 70% Ethyl alcohol or Isopropyl alcohol). It acts by denaturing proteins and dissolving lipid bilayers. While it is not sporicidal, it is a widely used antiseptic and disinfectant. **NEET-PG High-Yield Pearls:** * **Ethylene Oxide (EtO):** Highly effective but requires long aeration times due to toxicity and carcinogenicity. * **Alcohol:** Most effective at **60-90% concentration**; 100% absolute alcohol is less effective as it lacks the water required for protein denaturation. * **Spores:** Dry heat and EtO are sporicidal; standard alcohols are **not** sporicidal. * **Prions:** Most resistant to sterilization; require autoclaving at 134°C for 18 minutes with Sodium Hydroxide.

Question 167: Tyndallization is a method of

A. Growing bacteria in pure culture
B. Sterilization (Correct Answer)
C. Inoculation of media
D. Preserving cultures

Explanation: **Explanation:** **Tyndallization** (also known as fractional sterilization or intermittent sterilization) is a process designed to achieve **sterilization**—the complete destruction of all forms of microbial life, including highly resistant bacterial spores. ### Why it is Sterilization: The method involves heating the substance at **100°C for 20 minutes on three successive days**. * **Day 1:** The first heating kills vegetative bacteria but spares spores. * **Interval:** During the subsequent incubation at room temperature, the surviving spores germinate into vulnerable vegetative cells. * **Day 2 & 3:** Subsequent heating cycles kill these newly germinated vegetative cells. This repeated process ensures that even the most resistant spores are eliminated, achieving a sterile state. It is used for media containing ingredients that decompose at higher temperatures (like egg, serum, or sugars) which cannot withstand the high pressure of an autoclave. ### Why other options are incorrect: * **A & C (Growing/Inoculation):** These refer to the cultivation of microbes. Tyndallization is a physical method of control, not a technique for growth or introducing samples into media. * **D (Preserving cultures):** Preservation aims to keep microbes alive (e.g., lyophilization or cryopreservation). Tyndallization is intended to kill them. ### NEET-PG High-Yield Pearls: * **Temperature:** 100°C (Steam at atmospheric pressure). * **Equipment:** Koch’s or Arnold’s steam sterilizer. * **Key Use:** Sterilizing heat-sensitive media like **TCBS agar** or media containing **sugars/gelatin**. * **Comparison:** Unlike the **Autoclave** (121°C for 15 mins at 15 psi), Tyndallization does not use pressure. * **Inspissation:** Do not confuse with Tyndallization; Inspissation is used for high-protein media (LJ media) at 80-85°C for 30 mins for 3 days.

Question 168: What is the holding time for a hot air oven at 160°C?

A. 2 hours (Correct Answer)
B. 15 minutes
C. 30 minutes
D. 20 minutes

Explanation: **Explanation:** The **Hot Air Oven** is the most common method of **dry heat sterilization**. It works primarily through the oxidation of bacterial proteins and oxidative damage to components, leading to the death of microorganisms, including highly resistant spores. **1. Why 2 Hours is Correct:** For effective sterilization in a hot air oven, the standard "holding time" (the period the oven must maintain the specific temperature after reaching it) at **160°C is 120 minutes (2 hours)**. This duration is necessary because dry heat penetrates materials much more slowly than moist heat, requiring a longer exposure time to ensure the destruction of fungal and bacterial spores. **2. Analysis of Incorrect Options:** * **15 minutes (Option B):** This is the standard holding time for **Autoclaving** (moist heat) at 121°C (15 psi). Dry heat requires significantly longer than moist heat to achieve the same level of sterility. * **30 minutes (Option C):** This is the holding time for a hot air oven if the temperature is raised to **170°C**. At 160°C, 30 minutes is insufficient. * **20 minutes (Option D):** This is an incorrect duration for standard hot air oven protocols. **3. High-Yield Clinical Pearls for NEET-PG:** * **Sterilization Control:** The biological indicator used to check the efficacy of a hot air oven is **_Bacillus subtilis_ var. _niger_** (formerly *B. globigii*). * **What to Sterilize:** It is the method of choice for **glassware** (petri dishes, pipettes), **metallic instruments** (forceps, scalpels), and **anhydrous materials** (liquid paraffin, fats, dusting powders) that would be damaged by moisture. * **Temperature-Time Correlation:** * 160°C for 2 hours * 170°C for 1 hour * 180°C for 30 minutes * **Precaution:** Do not overload the oven; ensure free circulation of air. Do not open the oven until it cools down to below 60°C to prevent the cracking of glassware.

Question 169: What strength of phenol is used for the disinfection of feces?

A. 1%
B. 2.5%
C. 5%
D. 10% (Correct Answer)

Explanation: **Explanation:** **1. Why 10% is the Correct Answer:** Phenol (Carbolic acid) is a powerful disinfectant, but its efficacy is significantly reduced in the presence of organic matter. Feces contain high concentrations of organic material, proteins, and lipids that can neutralize lower concentrations of disinfectants. To ensure complete penetration and effective killing of pathogens (such as *Salmonella typhi* or *Vibrio cholerae*) within a bulky fecal mass, a higher concentration of **10% phenol** is required. A contact time of at least 1–2 hours is typically recommended for complete disinfection. **2. Analysis of Incorrect Options:** * **1% Phenol (Option A):** This concentration is too weak for heavy organic loads. It is generally used for simple bacteriostatic purposes or very light surface disinfection. * **2.5% Phenol (Option B):** While stronger, this remains insufficient for the dense organic matrix of feces. * **5% Phenol (Option C):** This is the standard concentration used for general disinfection of surfaces, instruments, and contaminated clothing. However, for specialized high-organic waste like feces or sputum, the concentration must be doubled to 10%. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rideal-Walker (RW) Coefficient:** Phenol is the "gold standard" against which other disinfectants are compared. The RW coefficient measures the killing power of a disinfectant compared to phenol. * **Mechanism of Action:** Phenols act by **disrupting cell membranes** and **precipitating cellular proteins**. * **Sputum Disinfection:** Like feces, sputum also requires **5–10% phenol** due to its high organic/mucus content. * **Limitation:** Phenols are generally ineffective against bacterial spores and non-enveloped viruses.

Question 170: A chest physician performs bronchoscopy in the procedure room of the out-patient department. To make the instrument safe for use in the next patient, what is the most appropriate method to disinfect the endoscope?

A. 70% alcohol for 5 minutes
B. 2% glutaraldehyde for 20 minutes (Correct Answer)
C. 2% formaldehyde for 10 minutes
D. 1% sodium hypochlorite for 15 minutes

Explanation: ### Explanation The core concept behind this question is the **Spaulding Classification of Medical Devices**. Bronchoscopes are classified as **semi-critical items** because they come into contact with mucous membranes or non-intact skin but do not penetrate sterile tissues. Such instruments require **High-Level Disinfection (HLD)**. **Why Option B is Correct:** **2% Glutaraldehyde (Cidex)** is the gold standard for HLD of flexible endoscopes. It is a dialdehyde that acts by alkylation of sulfhydryl, hydroxyl, carboxyl, and amino groups, effectively killing bacteria, mycobacteria, fungi, and viruses. A contact time of **20 minutes** is sufficient for HLD. It is preferred because it is non-corrosive to metals, rubber, and plastic lenses. **Why Other Options are Incorrect:** * **A. 70% Alcohol:** This is an intermediate-level disinfectant. While it kills vegetative bacteria and enveloped viruses, it lacks sporicidal activity and cannot reliably kill non-enveloped viruses or mycobacteria required for HLD. * **C. 2% Formaldehyde:** Although a high-level disinfectant, it is rarely used for endoscopes due to its pungent odor, slow action, and known **carcinogenic potential**. * **D. 1% Sodium Hypochlorite:** This is highly corrosive to the delicate metal components and optical fibers of endoscopes, leading to instrument damage. It is primarily used for surface disinfection (e.g., blood spills). **High-Yield Clinical Pearls for NEET-PG:** * **Sterilization vs. HLD:** To achieve "sterilization" (killing spores) with 2% Glutaraldehyde, an immersion time of **10 hours** is required. * **Ortho-phthalaldehyde (OPA):** A newer alternative to glutaraldehyde that is faster (12 mins) and doesn't require activation, though it is more expensive. * **Prions:** Glutaraldehyde is ineffective against prions; in fact, it may "fix" them to the instrument. * **Check for Potency:** The efficacy of glutaraldehyde must be monitored using test strips to ensure the concentration remains above the **Minimum Effective Concentration (MEC)** of 1.5%.

Practice by Chapter

Physical Methods of Sterilization

Practice Questions

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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