Sterilization and Disinfection Practice Questions

Q: The image shown depicts a piece of sterilization equipment. Which of the following statements regarding its sterilization properties is INCORRECT?

Prions are completely destroyed at 121°C for 15 minutes. ***Prions are completely destroyed at 121°C for 15 minutes*** - This statement is **incorrect** because **prions** are highly resistant to standard autoclaving protocols; they generally require more stringent methods such as **134°C** for 18 minutes in a pre-vacuum autoclave or specific chemical treatments like sodium hydroxide. - The standard cycle of **121°C for 15 minutes** is designed for vegetative bacteria, fungi, and viruses, but it is insufficient to denature the misfolded proteins associated with **prion diseases**. *It is a method of moist heat sterilization operating above 100°C* - The equipment shown is an **autoclave**, which utilizes **pressurized steam** to achieve temperatures significantly higher than the boiling point of water. - This elevated temperature effectively kills microorganisms through the **coagulation of cellular proteins** and enzymes, confirming the accuracy of this statement. *Bowie-Dick test positive* - While the test itself is designed to detect air leaks and ensure proper steam penetration for a successful cycle, saying

Q: Which agent is used for the sterilization of a cystoscope?

Glutaraldehyde. **Explanation:** The correct answer is **Glutaraldehyde (Option A)**. Cystoscopes are classified under **Spaulding’s Classification** as **semi-critical items** because they come into contact with mucous membranes but do not penetrate sterile tissue. These instruments require **High-Level Disinfection (HLD)**. **2% Glutaraldehyde (Cidex)** is the gold standard for heat-sensitive endoscopes. It acts by alkylation of amino, carboxyl, and hydroxyl groups, effectively killing bacteria, spores, fungi, and viruses. For HLD, an immersion time of **20 minutes** is standard, while **10 hours** is required for absolute sterilization (sporicidal action). **Why other options are incorrect:** * **Formaldehyde (B):** While a strong disinfectant, it is rarely used for endoscopes due to its pungent odor, irritating fumes, and potential carcinogenicity. It is primarily used for preserving tissues or fumigating rooms. * **Isopropyl alcohol (C):** This is a low-to-intermediate level disinfectant. It lacks sporicidal activity and can damage the lensed components and adhesives of a cystoscope. * **Ethylene oxide (D):** ETO is a method of sterilization for heat-sensitive items. However, it is a slow process requiring long aeration times to remove toxic residues, making it impractical for the rapid turnover required for cystoscopes in clinical practice. **High-Yield Clinical Pearls for NEET-PG:** * **Cidex Stability:** Once "activated" by adding an alkalizing agent, the solution is stable for only **14 days**. * **Ortho-phthalaldehyde (OPA):** A newer alternative to glutaraldehyde that is more stable and does not require activation, though it is more expensive. * **Plasma Sterilization (H2O2):** Increasingly used for modern robotic instruments and some endoscopes, but glutaraldehyde remains the most frequent answer for traditional "cold sterilization" questions.

Q: What is the biological indicator used for sterilization control in membrane filters?

Brevundimonas diminuta. ### Explanation **Correct Answer: D. Brevundimonas diminuta** **Concept:** Sterilization by filtration is used for heat-sensitive liquids (e.g., sera, antibiotics, vaccines). Unlike heat or chemical sterilization, filtration works by physical exclusion based on pore size. The standard pore size for "sterilizing grade" filters is **0.22 μm**. To validate these filters, a biological challenge is required using an organism small enough to test the filter's integrity. **_Brevundimonas diminuta_** (formerly *Pseudomonas diminuta*) is the organism of choice because it is a small, Gram-negative rod (approx. 0.3 μm in diameter) that can reliably penetrate filters with defects or larger-than-rated pores. **Analysis of Incorrect Options:** * **A. Bacillus pumilus:** This is the biological indicator used for **Ionizing Radiation** (Gamma rays). * **B. Bacillus stearothermophilus:** This is the most common indicator for **Autoclaving** (Moist heat) and Plasma sterilization, as its spores are highly heat-resistant. * **C. Clostridium tetani:** While it forms spores, it is not used as a standardized biological indicator for any sterilization process. **High-Yield Clinical Pearls for NEET-PG:** * **Ethylene Oxide (ETO):** Biological indicator is *Bacillus atrophaeus* (formerly *B. subtilis var. niger*). * **Dry Heat (Hot Air Oven):** Biological indicator is *Bacillus atrophaeus* (formerly *B. subtilis var. globigii*). * **HEPA Filters:** Efficiency is tested using the **DOP (Dioctyl Phthalate) test**, which uses particles of 0.3 μm. * **Filtration types:** Seitz filters (asbestos), Chamberland/Pasteur filters (porcelain), and Membrane filters (cellulose acetate/nitrate).

Q: Which of the following agents is effective in killing spores?

Formaldehyde. **Explanation:** The ability to kill bacterial spores (highly resistant, dormant structures) is the defining characteristic of **High-Level Disinfectants (HLD)** and **Sterilants**. **1. Why Formaldehyde is correct:** Formaldehyde is a potent high-level disinfectant and chemosterilant. It acts by **alkylation** of amino, carboxyl, and hydroxyl groups in nucleic acids and proteins, effectively cross-linking them. This irreversible damage is powerful enough to penetrate the thick coat of bacterial spores (e.g., *Bacillus* and *Clostridium* species), leading to their destruction. In gaseous form (fumigation), it is used to sterilize OT theaters and wards. **2. Why the other options are incorrect:** * **Alcohol (70% Ethyl/Isopropyl):** These are intermediate-level disinfectants. They act by denaturing proteins and dissolving lipids but are **not sporicidal** because they cannot penetrate the spore coat. * **Phenol:** Phenols act by disrupting cell membranes and precipitating proteins. While effective against vegetative bacteria and fungi, they are **not sporicidal**. * **Chlorine:** While sodium hypochlorite (bleach) has some sporicidal activity at high concentrations and long contact times, it is generally classified as an intermediate-level disinfectant and is less reliable than formaldehyde for total sterilization. **Clinical Pearls for NEET-PG:** * **Sporicidal Agents (The "Big 4"):** Glutaraldehyde (2% Cidex), Formaldehyde, Ethylene Oxide (ETO), and Hydrogen Peroxide (Plasma sterilization). * **Glutaraldehyde:** Known as the "Cold Sterilant" of choice for endoscopes (requires 10 hours for sporicidal action). * **Prions:** Most resistant to sterilization; require autoclaving at 134°C for 1 hour with 1N NaOH. * **Order of Resistance:** Prions > Spores > Mycobacteria > Non-enveloped viruses > Fungi > Vegetative bacteria > Enveloped viruses (HIV/HBV).

Question 1

The image shown depicts a piece of sterilization equipment. Which
of the following statements regarding its sterilization properties is
INCORRECT?

Accepted Answer

Prions are completely destroyed at 121°C for 15 minutes

Answer

It is a method of moist heat sterilization operating above 100°C

Answer

Bowie-Dick test positive

Answer

It is commonly used for sterilizing culture media

Question 2

A 30-year-old male with a 3-month history of evening fever, night sweats, 8 kg weight loss, and haemoptysis presents to a chest clinic. He is HIV-negative. A direct sputum smear is prepared and stained (Image 2). The smear shows ≥10 acid-fast bacilli per oil immersion field averaged over 20 fields. Based on this finding, which of the following statements regarding the smear result and its clinical significance is correct?

Accepted Answer

A. The smear grade indicates ≥10 bacilli per field averaged over 20 fields, signifying high bacillary load and high infectivity

Answer

B. The smear grade indicates 1–9 bacilli per 100 fields, classified as scanty and requiring repeat testing before treatment initiation

Answer

C. The smear grade indicates 10–99 bacilli per 100 fields, classified as Grade 1+ and sufficient for treatment initiation

Answer

D. The smear grade indicates 1–10 bacilli per field averaged over 50 fields, classified as Grade 2+ and correlating with moderate bacillary load

Question 3

Which agent is used for the sterilization of a cystoscope?

Accepted Answer

Glutaraldehyde

Answer

Formaldehyde

Answer

Isopropyl alcohol

Answer

Ethylene oxide

Question 4

What is the biological indicator used for sterilization control in membrane filters?

Accepted Answer

Brevundimonas diminuta

Answer

Bacillus pumilis

Answer

Bacillus stearothermophilus

Answer

Clostridium tetani

Question 5

Which of the following agents is effective in killing spores?

Accepted Answer

Formaldehyde

Answer

Alcohol

Answer

Phenol

Answer

Chlorine

Sterilization and Disinfection — MCQs

Sterilization and Disinfection — MCQs

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