Sterilization and Disinfection Practice Questions

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.

Q: How is a proctoscope typically sterilized?

Glutaraldehyde. **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).

Q: Holder method of Pasteurization does not kill which of the following microorganisms?

Coxiella burnetii. **Explanation:** The **Holder method** of pasteurization involves heating milk to **63°C (145°F) for 30 minutes**, followed by rapid cooling. The primary goal of pasteurization is to eliminate common milk-borne pathogens and spoilage organisms. **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. It can survive the standard Holder method (63°C for 30 mins) because its thermal death point is slightly higher. Consequently, the **Flash method** (High-Temperature Short-Time - HTST), which heats milk to **72°C for 15 seconds**, was specifically designed to ensure the destruction of *Coxiella burnetii*. **Analysis of Incorrect Options:** * **A. Mycobacteria:** *Mycobacterium bovis* and *Mycobacterium tuberculosis* were historically the primary targets of pasteurization. They are successfully killed by the Holder method. * **B. Brucella:** Species like *Brucella abortus* are highly heat-sensitive and are effectively eliminated at 63°C. * **C. Salmonella:** *Salmonella typhi* and other species are non-spore-forming Gram-negative rods that are easily killed by the temperatures used in the Holder method. **High-Yield Clinical Pearls for NEET-PG:** * **Phosphatase Test:** This is used to check the efficacy of pasteurization. Since the enzyme alkaline phosphatase is naturally present in milk and is destroyed at temperatures slightly higher than those required to kill most pathogens, its absence indicates successful pasteurization. * **Flash Method (HTST):** 72°C for 15 seconds. * **Ultra-High Temperature (UHT):** 135°C for 1-2 seconds (renders milk commercially sterile). * **Coxiella burnetii** is an obligate intracellular bacterium and is considered a potential bioterrorism agent (Category B).

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

Disinfectants. ### 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.

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

Bacillus stearothermophilus. **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.

Q: Gamma radiations are used for sterilizing which of the following medical supplies?

Syringes. **Explanation:** Gamma radiation is a form of **ionizing radiation** characterized by high penetrability and high energy. It acts by damaging microbial DNA and generating free radicals that disrupt cellular components. This method is often referred to as **"Cold Sterilization"** because it does not involve heat, making it ideal for heat-sensitive, pre-packaged medical supplies. * **Why Syringes are correct:** Disposable plastic items like syringes, catheters, and sutures are thermolabile (heat-sensitive). Gamma radiation (usually from a Cobalt-60 source) can penetrate the final commercial packaging to ensure absolute sterility without melting or damaging the plastic. * **Why other options are incorrect:** * **Cystoscopes:** These are delicate optical instruments. While some modern ones are autoclavable, they are traditionally sterilized using **Glutaraldehyde (2%)** or Ethylene Oxide (EtO) to prevent damage to the lenses and adhesives. * **Dressing aprons:** These are typically made of fabric or rubber. Fabric dressings are best sterilized using **Saturated Steam (Autoclave)**, which is more cost-effective and efficient for porous loads. * **Metal instruments:** The gold standard for surgical steel instruments is **Autoclaving** (121°C for 15-20 mins) or **Hot Air Oven** (160°C for 2 hours), as they can easily withstand high temperatures. **High-Yield Clinical Pearls for NEET-PG:** * **Source:** Cobalt-60 is the most common source of Gamma rays used in commercial sterilization plants. * **Dose:** The standard recommended dose for sterilization is **2.5 megarads (25 kGy)**. * **Bacillus pumilus:** This is the biological indicator used to test the efficacy of ionizing radiation. * **Cold Sterilization:** Remember this term specifically refers to Gamma radiation and sometimes Ethylene Oxide (EtO).

Q: Culture media containing egg or serum is typically sterilized by using which method?

Tyndallization. **Explanation:** The correct answer is **Tyndallization** (Intermittent Sterilization). **1. Why Tyndallization is correct:** Culture media containing heat-sensitive organic constituents like **egg** (e.g., Lowenstein-Jensen medium) or **serum** (e.g., Loeffler’s serum slope) cannot withstand the high temperatures of autoclaving (121°C), which would cause protein coagulation and nutrient degradation. Tyndallization involves heating the medium at **100°C for 20–30 minutes on three successive days**. * **Day 1:** Kills vegetative forms. * **Interval:** Remaining spores germinate into vegetative forms. * **Day 2 & 3:** Kills the newly germinated vegetative bacteria. **2. Why other options are incorrect:** * **Autoclaving:** Uses pressurized steam at 121°C. While it is the "gold standard" for most media, it destroys the texture and nutritional value of egg and serum-based media. * **Hot Air Oven:** Uses dry heat (160°C). This is used for glassware and forceps; it would char or incinerate organic culture media. * **5% Cresol:** This is a chemical disinfectant used for surfaces and discarded biological waste, not for sterilizing growth media. **3. NEET-PG High-Yield Pearls:** * **Inspiration:** If the temperature is kept between **75°C–85°C** for 3 successive days (specifically for LJ medium), the process is called **Inspissation**. * **Membrane Filtration:** Preferred for antibiotic solutions, urea, and vitamins that are highly thermolabile. * **Flash Point:** Remember that serum is sterilized at 56°C for one hour for several days if Tyndallization is not used. * **LJ Medium:** Sterilized via Inspissator at 80°C for 2 hours for 3 days.

Question 1

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

Accepted Answer

Bronchoscopes

Answer

Stethoscopes

Answer

Electronic thermometers

Answer

Surgical instruments

Question 2

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

Question 3

How is a proctoscope typically sterilized?

Accepted Answer

Glutaraldehyde

Answer

Autoclave

Answer

Formalin

Answer

Soap

Question 4

Holder method of Pasteurization does not kill which of the following microorganisms?

Accepted Answer

Coxiella burnetii

Answer

Mycobacteria

Answer

Brucella

Answer

Salmonella

Question 5

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

Accepted Answer

Disinfectants

Answer

Moist heat sterilization

Answer

Antibiotics

Answer

Dry heat sterilization

Question 6

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

Accepted Answer

Bacillus stearothermophilus

Answer

Bacillus subtilis

Answer

Staphylococcus aureus

Answer

Clostridium tetani

Question 7

Gamma radiations are used for sterilizing which of the following medical supplies?

Accepted Answer

Syringes

Answer

Cystoscopes

Answer

Dressing aprons

Answer

Metal instruments

Question 8

Laryngoscopes are sterilized by which method?

Accepted Answer

Glutaraldehyde

Answer

Formalin

Answer

Betadine

Answer

Boiling

Question 9

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?

Accepted Answer

2% glutaraldehyde for 20 minutes

Answer

70% alcohol for 5 minutes

Answer

2% formaldehyde for 10 minutes

Answer

1% sodium hypochlorite for 15 minutes

Question 10

Culture media containing egg or serum is typically sterilized by using which method?

Accepted Answer

Tyndallization

Answer

Autoclaving

Answer

Hot air oven

Answer

5% Cresol

Sterilization and Disinfection — MCQs

Sterilization and Disinfection — MCQs

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