Sterilization and Disinfection — MCQs

Sterilization and Disinfection Indian Medical PG Practice Questions and MCQs

Question 21: Sputum can be disinfected by all of the following methods, EXCEPT:

A. Boiling
B. Autoclaving
C. Cresol
D. Chlorhexidine (Correct Answer)

Explanation: **Explanation:** The correct answer is **Chlorhexidine** because it is a low-level disinfectant that is ineffective against *Mycobacterium tuberculosis*, the primary pathogen of concern in sputum. **1. Why Chlorhexidine is the correct answer:** Chlorhexidine is a biguanide antiseptic. While it is effective against most Gram-positive and some Gram-negative bacteria, it is **not mycobactericidal, sporicidal, or virucidal** (against non-enveloped viruses). Sputum often contains high organic loads and tubercle bacilli, which have a waxy, lipid-rich cell wall that chlorhexidine cannot penetrate. **2. Why the other options are incorrect:** * **Boiling:** Boiling at 100°C for 20–30 minutes kills most vegetative forms of bacteria, including *M. tuberculosis*. It is a standard physical method for disinfecting contaminated glassware or sputum containers in resource-limited settings. * **Autoclaving:** This is the "gold standard" for sterilization. Steam under pressure (121°C for 15 mins) ensures the destruction of all microorganisms, including highly resistant spores and mycobacteria. It is the preferred method for disposing of infectious laboratory waste. * **Cresol (Lysol):** Phenolic compounds like Cresol (5%) are highly effective against mycobacteria. They remain active even in the presence of organic matter (like mucus in sputum), making them the chemical disinfectant of choice for sputum spills or containers. **Clinical Pearls for NEET-PG:** * **Disinfectant of choice for Sputum:** 5% Cresol or 1% Sodium Hypochlorite (if organic load is low). * **Burning/Incineration:** The most preferred method for final disposal of sputum cups and contents. * **Chlorhexidine (Savlon/Hibitane):** Primarily used for skin antisepsis and preoperative hand scrubs; it is inactivated by soap and anionic detergents. * **Mycobacterium** is highly resistant to many disinfectants due to its **Mycolic acid** content.

Question 22: The process of destroying all life including spores is known as?

A. Sterilization (Correct Answer)
B. Disinfection
C. Antisepsis
D. Incineration

Explanation: ### Explanation **1. Why Option A (Sterilization) is Correct:** Sterilization is defined as the process by which all living microorganisms, including viable forms such as **bacterial spores**, viruses, and fungi, are completely eliminated or destroyed. In microbiology, the ability to kill spores is the "gold standard" for sterilization because spores are the most resistant forms of life. Common methods include autoclaving (moist heat), hot air oven (dry heat), and ethylene oxide (gas). **2. Why the Other Options are Incorrect:** * **B. Disinfection:** This process reduces the number of pathogenic microorganisms to a level that is no longer harmful, but it **does not typically kill bacterial spores**. It is used on inanimate objects. * **C. Antisepsis:** This refers to the application of a chemical agent to **living tissue** (like skin) to inhibit or destroy microbes. Like disinfection, it is generally ineffective against spores. * **D. Incineration:** While incineration is a *method* of sterilization (it destroys everything by burning it to ashes), the question asks for the *process* name. Incineration is a subset of sterilization used specifically for biomedical waste. **3. NEET-PG High-Yield Clinical Pearls:** * **The Spore Test:** *Geobacillus stearothermophilus* spores are used as the biological indicator to check the efficacy of autoclaves. * **Prions:** Standard sterilization may not destroy prions (infectious proteins); they require specific protocols like autoclaving at 134°C for 18 minutes in 1N NaOH. * **Disinfectant Levels:** * *High-level:* Kills all except high numbers of spores (e.g., Glutaraldehyde). * *Intermediate-level:* Kills mycobacteria and most viruses. * *Low-level:* Kills most bacteria but not spores or mycobacteria.

Question 23: Tyndallization involves steaming at what temperature and duration for effective sterilization?

A. 100°C for 20 minutes for 3 consecutive days (Correct Answer)
B. 121°C for 15 minutes for 3 consecutive days
C. 126°C for 2 minutes for 3 consecutive days
D. Less than 100°C for 20 minutes

Explanation: **Explanation:** **Tyndallization**, also known as fractional sterilization or intermittent sterilization, is a method used for materials that are damaged by the high heat of an autoclave (e.g., media containing serum, egg, or sugars). 1. **Why Option A is correct:** The process involves exposure to **steaming at 100°C for 20 minutes on three successive days**. The underlying principle is that the first heating kills all vegetative bacteria. During the subsequent incubation periods (intervals between heating), any resistant **spores** germinate into vegetative forms, which are then killed during the second and third heating sessions. 2. **Why other options are incorrect:** * **Option B & C:** These temperatures (121°C and 126°C) refer to **Autoclaving** (moist heat under pressure). Autoclaving is a single-cycle process and does not require three consecutive days. * **Option D:** Heating at less than 100°C (e.g., 63°C or 72°C) refers to **Pasteurization**, which is a disinfection process, not a sterilization method, as it does not reliably kill spores. **High-Yield Clinical Pearls for NEET-PG:** * **Principle:** Tyndallization relies on the **germination of spores** into vulnerable vegetative cells. * **Equipment:** It is performed in a **Koch’s or Arnold’s steamer**. * **Comparison:** Unlike the Autoclave (which uses 15 psi pressure), Tyndallization uses **atmospheric pressure**. * **Inspissation:** Do not confuse Tyndallization with Inspissation (80-85°C for 30 mins for 3 days), which is used specifically for media like LJ medium or Loeffler’s serum slope.

Question 24: What is cold sterilization?

A. Ionizing radiation (Correct Answer)
B. Mechanical filtration
C. Ultrasonic cleaning
D. Sonic vibrations with heating and sudden cooling

Explanation: **Explanation:** **Cold Sterilization** refers to the process of sterilization without the use of heat. This is crucial for heat-sensitive materials that would otherwise be damaged by autoclaving or dry heat. **Why Ionizing Radiation is the Correct Answer:** Ionizing radiations, such as **Gamma rays** (from Cobalt-60) and **X-rays**, possess high energy and deep penetrative power. They kill microorganisms by inducing lethal mutations in their DNA and generating free radicals. Since this process does not involve a significant rise in temperature, it is termed "Cold Sterilization." It is the method of choice for sterilizing pre-packed disposable items like plastic syringes, catheters, and sutures. **Analysis of Incorrect Options:** * **B. Mechanical Filtration:** This is a method of physical separation, not destruction. While it removes bacteria from heat-labile liquids (like sera or antibiotics), it does not kill them and often fails to remove smaller viruses or mycoplasma. * **C. Ultrasonic Cleaning:** This uses high-frequency sound waves to create cavitation bubbles that dislodge organic debris from instruments. It is a **cleaning** step, not a sterilization process. * **D. Sonic vibrations with heating/cooling:** This describes a combination of physical agitation and thermal stress, which does not define the specific medical term "cold sterilization." **High-Yield Clinical Pearls for NEET-PG:** * **Gamma Radiation:** Most common source is **Cobalt-60**. Used for "disposable" medical supplies. * **Non-ionizing radiation (UV rays):** Used for surface disinfection and air in OTs; lacks penetrative power. * **Chemical Cold Sterilization:** 2% **Glutaraldehyde** (Cidex) is used for endoscopes (requires 10 hours for sporicidal action). * **Ethylene Oxide (ETO):** Another form of cold sterilization used for heart-lung machines and respirators.

Question 25: What is incineration?

A. A high temperature oxidation process (Correct Answer)
B. A low temperature reduction process
C. A high temperature reduction process
D. A low temperature oxidation process

Explanation: **Explanation:** **Incineration** is a method of dry heat sterilization that involves the combustion of organic substances contained in waste materials. 1. **Why Option A is Correct:** Incineration is fundamentally a **high-temperature oxidation** process. It operates at very high temperatures (typically ranging from 870°C to 1200°C). During this process, waste is converted into ash, flue gas, and heat. The organic carbon in the waste reacts with oxygen (oxidation) to form carbon dioxide and water vapor, effectively reducing the volume of the waste by up to 90%. 2. **Why Other Options are Incorrect:** * **Options B & D (Low Temperature):** Incineration requires extreme heat to ensure complete destruction of pathogens and anatomical structures; low temperatures would result in incomplete combustion and survival of infectious agents. * **Options B & C (Reduction):** Reduction is the gain of electrons or loss of oxygen. Incineration relies on the presence of excess oxygen to "burn" the waste; therefore, it is an oxidative, not a reductive, process. **High-Yield Clinical Pearls for NEET-PG:** * **Best Method for:** Incineration is the gold standard for disposing of **Anatomical Waste** (human tissues, organs, body parts) and **Cytotoxic drugs**. * **BMW Management:** According to Biomedical Waste Management rules, waste in **Yellow Bags** is primarily disposed of via incineration. * **Contraindication:** Never incinerate **PVC (Polyvinyl Chloride)** plastics (like IV sets or blood bags) as it releases toxic **dioxins and furans** into the atmosphere. * **End Product:** The process results in "sterile ash," which is then disposed of in secured landfills.

Question 26: What is the method of choice for sterilization of liquid paraffin?

A. Flaming
B. Moist heat
C. Autoclave
D. Hot air oven (Correct Answer)

Explanation: **Explanation:** The method of choice for sterilizing **liquid paraffin** (and other oily substances) is the **Hot Air Oven (Dry Heat)**. **Why Hot Air Oven is correct:** Liquid paraffin, fats, oils, and powders are **anhydrous (water-free) substances**. Moist heat (autoclaving) is ineffective for these materials because steam cannot penetrate through oils or grease to reach the microorganisms. Dry heat sterilization works by causing **oxidative damage** to microbial components and denaturation of proteins. It is the only effective method for substances that are impermeable to or damaged by moisture. The standard cycle for this is **160°C for 2 hours**. **Why the other options are incorrect:** * **Flaming (A):** This is used for inoculating loops, wires, and points of forceps. It is impractical and dangerous for bulk liquids like paraffin due to the risk of combustion. * **Moist Heat/Autoclave (B & C):** Autoclaving uses saturated steam under pressure. Since oil and water do not mix, the steam cannot penetrate the oil to achieve sterilization. Furthermore, moisture can cause the paraffin to become turbid or milky. **High-Yield Clinical Pearls for NEET-PG:** * **Sterilization of choice for Glassware:** Hot Air Oven. * **Sterilization of choice for Talcum Powder:** Hot Air Oven. * **Sterilization of choice for Sharp Instruments:** Hot Air Oven (to prevent rusting of the cutting edge, though many modern guidelines prefer autoclaving for surgical sets). * **Biological Indicator for Hot Air Oven:** *Bacillus atrophaeus* (formerly *B. subtilis var. niger*). * **Biological Indicator for Autoclave:** *Geobacillus stearothermophilus*.

Question 27: Heart-lung machines are typically sterilized by which method?

A. Orthophthaldehyde
B. Cidex
C. Isopropyl alcohol
D. Ethylene oxide (Correct Answer)

Explanation: **Explanation:** **Ethylene Oxide (EtO)** is the method of choice for sterilizing heart-lung machines and other complex medical equipment because it is a **"cold sterilization"** process. Heart-lung machines contain heat-sensitive components, such as delicate plastics, electronics, and optical lenses, which would be damaged by the high temperatures of an autoclave. EtO works by **alkylation** of amino, carboxyl, and hydroxyl groups in microbial proteins and nucleic acids, effectively killing all microorganisms, including highly resistant bacterial spores. Its high penetrability allows it to reach the intricate internal tubing and narrow lumens characteristic of bypass equipment. **Analysis of Incorrect Options:** * **Orthophthaldehyde (OPA) & Cidex (2% Glutaraldehyde):** These are high-level disinfectants (HLDs) primarily used for semi-critical items like endoscopes. While effective, they are liquid chemicals; immersing a complex heart-lung machine in liquid is impractical and can leave toxic residues if not rinsed perfectly. * **Isopropyl alcohol:** This is an intermediate-level disinfectant used mainly for skin antisepsis or surface disinfection. It is not sporicidal and therefore cannot achieve the level of sterilization required for equipment entering the vascular system. **Clinical Pearls for NEET-PG:** * **EtO Monitoring:** The efficacy of EtO is monitored biologically using **_Bacillus atrophaeus_** (formerly *B. subtilis var. niger*) spores. * **Aeration:** Post-sterilization, items must be aerated to remove residual EtO gas, which is mutagenic and irritating to tissues. * **Other EtO uses:** Ideal for pacemakers, artificial heart valves, and disposable syringes.

Question 28: An autoclave sterilizes dental instruments by causing which of the following?

A. Coagulation and denaturation of proteins. (Correct Answer)
B. Oxidative damage.
C. Precipitation of nucleic acids.
D. Disruption of cell membranes.

Explanation: **Explanation:** The autoclave operates on the principle of **moist heat sterilization** (saturated steam under pressure). The primary mechanism of microbial destruction by moist heat is the **coagulation and denaturation of structural and enzymatic proteins**. Water molecules facilitate the breaking of hydrogen bonds that hold the three-dimensional structure of proteins together. Once these proteins are denatured and coagulated, the cell loses its functional integrity, leading to irreversible microbial death, including the destruction of highly resistant bacterial spores. **Analysis of Incorrect Options:** * **B. Oxidative damage:** This is the primary mechanism of **Dry Heat sterilization** (e.g., Hot Air Oven). Dry heat causes death by oxidation of cell constituents and damage to proteins through dehydration. * **C. Precipitation of nucleic acids:** While some chemicals may affect nucleic acids, it is not the primary mechanism of sterilization for autoclaving. * **D. Disruption of cell membranes:** This is the characteristic mechanism of action for many **disinfectants**, such as phenols, alcohols, and quaternary ammonium compounds, rather than high-pressure steam. **NEET-PG High-Yield Pearls:** * **Standard Autoclave Settings:** 121°C at 15 psi (pounds per square inch) for 15–20 minutes. * **Sterilization Control (Biological Indicator):** Spores of ***Geobacillus stearothermophilus*** (formerly *Bacillus stearothermophilus*). * **Flash Sterilization:** 134°C for 3 minutes (used for urgent dental/surgical instruments). * **Prion Sterilization:** Requires higher parameters (134°C for 1 hour or 121°C for 90 minutes).

Question 29: Tyndallization is a method of sterilization using?

A. Moist heat above 100 C
B. Moist heat below 100 C
C. Dry heat above 100 C
D. Moist heat at 100 C (Correct Answer)

Explanation: ### Explanation **Tyndallization**, also known as intermittent or fractional sterilization, is a method of sterilization using **moist heat at 100°C**. **Why Option D is Correct:** The process involves heating the medium at **100°C for 20–30 minutes on three successive days**. * **Day 1:** The first heating kills all vegetative forms of bacteria. * **Interval:** During the subsequent incubation at room temperature, any resistant spores present germinate into vegetative forms. * **Day 2 & 3:** These newly germinated vegetative cells are killed by the subsequent heating cycles. This method is specifically used for media containing ingredients that are decomposed by higher temperatures in an autoclave (e.g., egg, serum, or sugars). **Why Other Options are Incorrect:** * **Option A (Moist heat >100°C):** This refers to **Autoclaving** (121°C for 15 mins at 15 psi), which is the standard for surgical instruments and most culture media. * **Option B (Moist heat <100°C):** This refers to **Pasteurization** (63°C or 72°C) or **Inspissation** (80-85°C), used for milk and Lowenstein-Jensen (LJ) media respectively. * **Option C (Dry heat >100°C):** This refers to the **Hot Air Oven** (160°C for 2 hours), used for glassware and oily substances. **High-Yield Clinical Pearls for NEET-PG:** * **Principle:** Tyndallization relies on the **germination of spores** to make them vulnerable to heat. * **Equipment:** It is performed in a **Koch’s or Arnold’s steam sterilizer**. * **Key Use:** Sterilization of sugar media and gelatin media. * **Limitation:** It is not reliable against certain thermophilic spores or anaerobic spores if the medium does not support their germination between cycles.

Question 30: Sunlight acts as a sterilizing agent due to which property?

A. Long wavelength radiation
B. Short wavelength radiation (Correct Answer)
C. Presence of ozone
D. Generation of heat

Explanation: **Explanation:** The sterilizing property of sunlight is primarily attributed to its **short wavelength radiation**, specifically the **Ultraviolet (UV) rays**. Among these, UV-C (200–280 nm) is the most germicidal, followed by UV-B. These short wavelengths carry high energy that is absorbed by the microbial DNA, leading to the formation of **pyrimidine (thymine) dimers**. This structural damage inhibits DNA replication and transcription, ultimately causing cell death. **Analysis of Options:** * **A. Long wavelength radiation:** Infrared and visible light have longer wavelengths and lower energy. While they contribute to heat, they do not possess the energy required to cause direct photochemical damage to microbial DNA. * **C. Presence of ozone:** While the ozone layer filters out the most lethal short-wave UV rays (UV-C) from reaching the earth's surface, it is the radiation itself that kills microbes, not the ozone gas. * **D. Generation of heat:** While sunlight can cause desiccation (drying), the primary "sterilizing" or bactericidal action is photochemical (DNA damage) rather than thermal. **High-Yield Clinical Pearls for NEET-PG:** * **UV Radiation:** Known as "non-ionizing radiation." It has low penetrating power; therefore, it is used for disinfecting surfaces, air (in OTs), and clear water, but not for deep sterilization. * **Ionizing Radiation:** Includes Gamma rays and X-rays (shorter wavelength than UV). These have high penetrating power and are used for "Cold Sterilization" of heat-sensitive disposable items like plastic syringes, catheters, and sutures. * **DNA Damage:** Remember that the specific mechanism for UV light is **Thymine Dimer formation**, a frequent topic in both Microbiology and Biochemistry.

Practice by Chapter

Physical Methods of Sterilization

Practice Questions

Chemical Methods of Sterilization

Practice Questions

Disinfectants and Antiseptics

Practice Questions

Sterilization Monitoring

Practice Questions

Hospital Sterilization Protocols

Practice Questions

Surgical Instruments Sterilization

Practice Questions

Endoscope Reprocessing

Practice Questions

Biological Indicators

Practice Questions

Factors Affecting Sterilization

Practice Questions

Quality Control in Sterilization

Practice Questions

Biofilm and Its Implications

Practice Questions

Regulatory Aspects of Sterilization

Practice Questions

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