Sterilization and Disinfection Practice Questions

Q: Which bacterium is used as an indicator for dry heat sterilization?

Bacillus subtilis. **Explanation:** Sterilization monitoring is a high-yield topic in Microbiology. To ensure a sterilization process is effective, biological indicators (spores of specific non-pathogenic bacteria) are used because they are more resistant to heat and chemicals than most pathogens. **1. Why Bacillus subtilis is correct:** * **Bacillus subtilis (subspecies niger):** This is the standard biological indicator for **Dry Heat Sterilization** (Hot Air Oven) and Ethylene Oxide (EtO) gas sterilization. These spores are highly resistant to desiccation and dry heat, making them the ideal "challenge" organism to verify that the oven has reached the required temperature for the necessary duration. **2. Analysis of Incorrect Options:** * **Bacillus stearothermophilus (Geobacillus stearothermophilus):** This is the indicator for **Moist Heat Sterilization (Autoclave)** and Plasma sterilization. It is thermophilic, meaning it thrives at high temperatures, and its spores are specifically resistant to pressurized steam. * **Bacillus pumilus:** This is used as the biological indicator for **Ionizing Radiation** (Gamma rays). * **Coxiella burnetii:** This is the most heat-resistant non-spore-forming pathogen found in milk. It is used as the indicator organism for the efficacy of **Pasteurization**, not for dry heat sterilization. **Clinical Pearls for NEET-PG:** * **Hot Air Oven:** Standard cycle is 160°C for 2 hours. * **Autoclave:** Standard cycle is 121°C at 15 psi for 15 minutes. * **Chick-Martin Test / Rideal-Walker Coefficient:** Used to grade disinfectants (Phenol coefficient). * **Flash Pasteurization:** 72°C for 15 seconds.

Q: Which method is used for sterilization by steam under pressure?

Autoclave. **Explanation:** The **Autoclave** is the gold standard for sterilization by **moist heat (steam) under pressure**. It operates on the principle that increasing the pressure in a closed vessel raises the boiling point of water, allowing steam to reach temperatures above 100°C. The standard cycle is **121°C at 15 psi for 15 minutes**, which is sufficient to kill all vegetative forms of bacteria, fungi, viruses, and, most importantly, highly resistant **bacterial spores**. **Analysis of Incorrect Options:** * **Tyndall’s Chamber / Tyndallization:** This is a method of intermittent steaming at 100°C for 20–30 minutes on three successive days. It is used for media containing heat-sensitive ingredients like sugar or gelatin. * **Koch’s Arnold Steamer:** This uses steam at atmospheric pressure (100°C) for 90 minutes. It is used for media that cannot withstand the high pressure of an autoclave. It does not reliably kill all spores in a single cycle. * **Hot Air Oven:** This is the primary method for **dry heat sterilization**. It operates at 160°C for 2 hours and is used for glassware, forceps, and oily substances that are impermeable to steam. **High-Yield Facts for NEET-PG:** * **Mechanism of Action:** Moist heat kills microorganisms by **denaturation and coagulation of structural proteins and enzymes**. * **Sterilization Control:** The biological indicator used to check the efficacy of an autoclave is **Geobacillus stearothermophilus** (formerly *Bacillus stearothermophilus*). * **Clinical Application:** Autoclaving is the preferred method for surgical instruments, culture media, and infectious waste. * **Flash Sterilization:** A rapid autoclaving method (134°C for 3 minutes) used for urgent surgical items.

Q: Which of the following is NOT a true statement regarding disinfectants?

Ethylene oxide is an intermediate disinfectant.. ### Explanation The correct answer is **C**, as Ethylene Oxide (EtO) is classified as a **high-level disinfectant/sterilant**, not an intermediate one. **1. Why Option C is the correct (False) statement:** Disinfectants are categorized into three levels based on their biocidal activity: * **High-level:** Kill all microbes, including bacterial spores (e.g., Glutaraldehyde, Ethylene Oxide, Hydrogen Peroxide). * **Intermediate-level:** Kill mycobacteria, vegetative bacteria, and most viruses/fungi, but **not** bacterial spores (e.g., Alcohols, Iodophors). * **Low-level:** Kill most vegetative bacteria and enveloped viruses (e.g., Quaternary ammonium compounds). **Ethylene Oxide** is a potent alkylating agent used for gaseous sterilization of heat-sensitive items (like heart-lung machines and catheters). It is highly effective against all microorganisms, including spores, making it a sterilant/high-level disinfectant. **2. Analysis of other options:** * **Option A (Glutaraldehyde is sporicidal):** This is **True**. 2% Glutaraldehyde (Cidex) is a high-level disinfectant. It requires 20 minutes for disinfection but 10 hours of immersion to be truly sporicidal ("cold sterilization"). * **Option B (Hypochlorites are virucidal):** This is **True**. Sodium hypochlorite is highly effective against viruses, including HIV and Hepatitis B. It is the disinfectant of choice for cleaning blood spills. **High-Yield Clinical Pearls for NEET-PG:** * **Glutaraldehyde (2%):** Used for endoscopes and bronchoscopes. Once activated, it has a shelf life of 14 days. * **Ethylene Oxide (EtO):** Best for plastic/rubber goods that cannot withstand heat. Its main drawback is toxicity and the requirement for long aeration periods. * **Blood Spills:** Use 1% Hypochlorite for small spills and 10% for large spills. * **Prions:** Most resistant to sterilization; require autoclaving at 134°C for 18 minutes or 1N NaOH for 1 hour.

Q: What is the typical temperature of an autoclave operating at 30 psi of pressure?

134°C. ### Explanation The autoclave operates on the principle of **moist heat sterilization** using saturated steam under pressure. The core concept is that increasing the pressure within a closed vessel raises the boiling point of water, allowing steam to reach temperatures higher than 100°C, which is necessary to kill highly resistant bacterial spores. **Why 134°C is correct:** Standard autoclaving typically follows two main protocols: 1. **15 psi at 121°C for 15–20 minutes:** This is the standard cycle for most laboratory media and surgical instruments. 2. **30 psi at 134°C for 3 minutes:** Known as "Flash Sterilization" or "High-speed sterilization," this higher pressure allows for a significantly higher temperature, reducing the required holding time. This is commonly used for unwrapped instruments in emergency clinical settings. **Analysis of Incorrect Options:** * **A. 121°C:** This is the temperature achieved at **15 psi**. While it is the most common autoclave setting, it does not correspond to 30 psi. * **B. 100°C:** This is the boiling point of water at sea level. Methods using 100°C include Tyndallization and steaming (Koch’s sterilizer), which are not pressurized. * **C. 63°C:** This temperature is used in **Holder Pasteurization** (63°C for 30 minutes) for milk, which is a disinfection process, not sterilization. **High-Yield Clinical Pearls for NEET-PG:** * **Sterilization Check:** The biological indicator of choice for autoclaving is **_Geobacillus stearothermophilus_** spores. * **Chemical Indicator:** **Browne’s tubes** (color change from red to green) or **Bowie-Dick test** (for vacuum efficiency). * **Mechanism of Action:** Moist heat kills microorganisms by **denaturation and coagulation of structural proteins and enzymes**. * **Prion Protocol:** For prion-contaminated instruments, autoclaving at **134°C for 1 hour** is recommended.

Q: Which of the following statements regarding the properties of common disinfectants is FALSE?

Phenol usually requires organic matter to act.. ### Explanation **1. Why Option D is the correct (False) statement:** The statement is false because **phenols are actually notable for being effective even in the presence of organic matter** (such as pus, blood, or feces). Unlike many other disinfectants (like halogens) that are inactivated by organic debris, phenolic compounds (e.g., Lysol, Cresol) maintain their efficacy. This property makes them particularly useful for disinfecting excreta and surfaces in clinical settings. **2. Analysis of Incorrect Options (True Statements):** * **A. Glutaraldehyde is sporicidal:** This is true. Glutaraldehyde (e.g., Cidex) is a high-level disinfectant. It is sporicidal after prolonged exposure (usually 10 hours) and is commonly used for "cold sterilization" of endoscopes. * **B. Hypochlorites are viricidal:** This is true. Hypochlorites (Bleach) are potent oxidizing agents effective against a wide range of viruses, including HBV and HIV. They are the disinfectant of choice for managing blood spills. * **C. Alcohol is fungicidal and bactericidal but not sporicidal:** This is true. 70% Ethyl or Isopropyl alcohol acts by denaturing proteins. While effective against vegetative bacteria, fungi, and enveloped viruses, it lacks the ability to penetrate and kill bacterial spores. **3. High-Yield Clinical Pearls for NEET-PG:** * **Chick-Martin Test:** Used to determine the efficacy of disinfectants in the presence of organic matter (specifically using sterilized feces). * **Rideal-Walker Coefficient:** Compares a disinfectant's efficiency to Phenol (Phenol Coefficient). * **Endoscope Disinfection:** 2% Glutaraldehyde is the standard; however, **Ortho-phthalaldehyde (OPA)** is a newer alternative that is faster and less irritating. * **Blood Spills:** Sodium hypochlorite (1% for small spills, 10% for large spills) is the gold standard.

Q: Which of the following is best for sterilizing heat-labile solutions?

Membrane filtration. **Explanation:** **Why Membrane Filtration is Correct:** Sterilization of **heat-labile (heat-sensitive)** solutions requires a method that does not involve high temperatures, which would otherwise denature proteins or degrade chemical compounds. Membrane filtration is a **physical method** that removes microorganisms (bacteria, fungi, and some large viruses) by passing the liquid through pores smaller than the microbes (typically **0.22 µm**). This is the gold standard for sterilizing sera, antibiotic solutions, vaccines, and enzyme preparations. **Why Other Options are Incorrect:** * **Dry Heat (Hot Air Oven):** Uses high temperatures (160°C for 2 hours). It is used for glassware and oil-based substances but would destroy heat-sensitive liquids. * **Autoclave (Moist Heat):** Uses saturated steam under pressure (121°C for 15 mins). While highly effective for surgical instruments and culture media, it causes irreversible damage to heat-labile proteins and vitamins. * **Pasteurization:** This is a **disinfection** process, not sterilization. It uses moderate heat to reduce the microbial load in milk and beverages but does not kill bacterial spores. **High-Yield Clinical Pearls for NEET-PG:** * **Pore Size:** A 0.22 µm filter removes most bacteria, but to remove smaller viruses or *Mycoplasma*, even smaller pore sizes are required. * **HEPA Filters:** A form of filtration used for **air sterilization** in OTs and laminar airflow hoods (removes particles >0.3 µm). * **Ethylene Oxide (EtO):** If the question asks for heat-labile **equipment** (like plastics or heart-lung machines) rather than solutions, EtO is the preferred gaseous sterilant. * **Flash Sterilization:** A rapid autoclaving method (134°C for 3 mins) used for urgently needed surgical instruments.

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

2% glutaraldehyde for 20 minutes. **Explanation:** The correct method for disinfecting flexible endoscopes (like bronchoscopes) is **High-Level Disinfection (HLD)** using **2% Glutaraldehyde (Cidex)** for a contact time of **20 minutes**. **Why 2% Glutaraldehyde is the Correct Choice:** Endoscopes are classified as **semi-critical items** according to Spaulding’s Classification because they come into contact with mucous membranes but do not penetrate sterile tissue. These items require HLD, which destroys all microorganisms except high numbers of bacterial spores. 2% Glutaraldehyde is the gold standard because it is non-corrosive to lenses, rubber, and metal, and it effectively kills bacteria, fungi, and viruses (including HIV and HBV) within 20 minutes. **Analysis of Incorrect Options:** * **A. 70% Alcohol:** This is an intermediate-level disinfectant. While it kills vegetative bacteria and some viruses, it is ineffective against spores and can damage the adhesive/lenses of the endoscope. * **C. 2% Formaldehyde:** While it is a high-level disinfectant, it is rarely used for endoscopes due to its pungent odor, irritating fumes, and potential carcinogenicity. It also requires much longer contact times. * **D. 1% Sodium Hypochlorite:** This is highly corrosive to the metal components and delicate channels of flexible endoscopes, making it unsuitable for this purpose. **High-Yield Clinical Pearls for NEET-PG:** * **Spaulding’s Classification:** Critical (Sterilization), Semi-critical (HLD), Non-critical (Low-level disinfection). * **Sterilization vs. Disinfection:** To achieve **sterilization** (sporicidal action) with 2% Glutaraldehyde, an immersion time of **10 hours** is required. * **Cidex Life:** Once activated, the 2% glutaraldehyde solution remains effective for **14 days**. * **Alternative:** **Ortho-phthalaldehyde (OPA)** is a newer alternative to glutaraldehyde that is faster (12 mins) and less irritating.

Question 1

Which one of the following disinfectants is not effective in killing spores?

Accepted Answer

Alcohol

Answer

Aldehyde

Answer

Halogen

Answer

Phenol

Question 2

Which bacterium is used as an indicator for dry heat sterilization?

Accepted Answer

Bacillus subtilis

Answer

Bacillus pumilis

Answer

Bacillus stearothermophilus

Answer

Coxiella burnetti

Question 3

Which method is used for sterilization by steam under pressure?

Accepted Answer

Autoclave

Answer

Tyndall's Chamber

Answer

Koch's Arnold Steamer

Answer

Hot air oven

Question 4

Which of the following is NOT a true statement regarding disinfectants?

Accepted Answer

Ethylene oxide is an intermediate disinfectant.

Answer

Gluteraldehyde is sporicidal.

Answer

Hypochlorites are virucidal.

Answer

All of the above statements are true.

Question 5

What is the typical temperature of an autoclave operating at 30 psi of pressure?

Accepted Answer

134°C

Answer

121°C

Answer

100°C

Answer

63°C

Question 6

Which of the following represents the most appropriate method for hand hygiene in a healthcare setting?

Accepted Answer

Cleaning hands with an alcohol-based hand rub before and after examining each patient.

Answer

Washing hands with soap and water when there is visible blood on the hands.

Answer

Using soap and Dettol exclusively for hand washing prior to surgery.

Answer

Hand rubbing with alcohol-based solutions is never a substitute for hand washing.

Question 7

Which of the following statements regarding the properties of common disinfectants is FALSE?

Accepted Answer

Phenol usually requires organic matter to act.

Answer

Glutaraldehyde is sporicidal.

Answer

Hypochlorites are viricidal.

Answer

Alcohol is fungicidal and bactericidal but not sporicidal.

Question 8

Which of the following is best for sterilizing heat-labile solutions?

Accepted Answer

Membrane filtration

Answer

Dry heat

Answer

Autoclave

Answer

Pasteurization

Question 9

A chest physician performs bronchoscopy in the procedure room of the outpatient 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

Irradiation can be used to sterilize which of the following?

Accepted Answer

Bronchoscope

Answer

Bone graft

Answer

Suture

Answer

Artificial tissue graft

Sterilization and Disinfection — MCQs

Sterilization and Disinfection — MCQs

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