Sterilization and Disinfection Practice Questions

Q: All of the following are sporicidal agents EXCEPT:

Ethyl and isopropyl alcohol. **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).

Q: Glutaraldehyde is used for sterilization of which of the following?

All of the above. **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).

Q: Irradiation can be used to sterilize all except:

Bronchoscope. **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.

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

2% glutaraldehyde. **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.

Q: What are the required conditions for effective autoclaving?

121°C temperature for 15 minutes. **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).

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

Tyndallization. ### 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.

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

Prions. **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.

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

Bronchoscopes. ### 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*.

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

Geobacillus stearothermophilus. ### 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.

Question 1

All of the following are sporicidal agents EXCEPT:

Accepted Answer

Ethyl and isopropyl alcohol

Answer

Bleaching powder and ethylene oxide

Answer

Iodine and chlorine compounds

Answer

Hydrogen peroxide and bleaching powder

Question 2

Glutaraldehyde is used for sterilization of which of the following?

Accepted Answer

All of the above

Answer

Endoscopes

Answer

Corrugated rubber anaesthetic tube

Answer

Plastic endotracheal tube

Question 3

Irradiation can be used to sterilize all except:

Accepted Answer

Bronchoscope

Answer

Bone graft

Answer

Suture

Answer

Artificial tissue graft

Question 4

Which of the following is used for the sterilization of laparoscopic instruments?

Accepted Answer

2% glutaraldehyde

Answer

2% formalin

Answer

Autoclaving

Answer

Boiling

Question 5

Which of the following is an intermediate level disinfectant?

Accepted Answer

Iodophores

Answer

Glutaraldehyde

Answer

Quaternary ammonium compound

Answer

Hydrogen peroxide

Question 6

What are the required conditions for effective autoclaving?

Accepted Answer

121°C temperature for 15 minutes

Answer

121°C temperature for 20 minutes

Answer

100°C temperature for 60 minutes

Answer

100°C temperature for 90 minutes

Question 7

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

Accepted Answer

Tyndallization

Answer

Autoclaving

Answer

Pasteurization

Answer

Inspissation

Question 8

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

Accepted Answer

Prions

Answer

Spores

Answer

Protozoa

Answer

Fungi

Question 9

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

Accepted Answer

Bronchoscopes

Answer

Stethoscopes

Answer

Electronic thermometers

Answer

Surgical instruments

Question 10

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

Accepted Answer

Geobacillus stearothermophilus

Answer

Bacillus subtilis

Answer

Clostridium tetani

Answer

Clostridium perfringens

Sterilization and Disinfection — MCQs

Sterilization and Disinfection — MCQs

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