Sterilization and Disinfection Practice Questions

Q: All of the following are used in the disinfection of faeces, EXCEPT:

Glutaraldehyde. **Explanation:** The disinfection of faeces (especially in cases of cholera or enteric fever) requires agents that are effective in the presence of high organic matter and are cost-efficient for large-scale waste management. **Why Glutaraldehyde is the correct answer:** Glutaraldehyde (Cidex) is a high-level disinfectant primarily used for **"cold sterilization" of delicate instruments** like endoscopes, cystoscopes, and bronchoscopes. It is expensive, requires a specific pH (activated at 8.0), and has a relatively short shelf life once activated. Using it for faecal disinfection is impractical, non-cost-effective, and clinically inappropriate. **Analysis of incorrect options:** * **Cresol (Lysol):** Phenolic compounds like cresol are the **standard choice** for faecal disinfection. They remain active in the presence of organic matter and are highly effective against vegetative bacteria. * **Bleaching Powder (Calcium Hypochlorite):** This is a common and effective disinfectant for stools and urine. It acts by releasing chlorine, which is a potent oxidizing agent. * **Formalin:** 10% Formalin is a powerful disinfectant used for solid waste and tissues. It acts by alkylation of proteins and nucleic acids. **NEET-PG High-Yield Pearls:** * **Standard for Faeces:** Cresol (5%) is the most preferred agent. * **Glutaraldehyde (2%):** Requires 20 minutes for disinfection and 10 hours for sterilization (sporicidal). It is the agent of choice for **Lensed Instruments**. * **Organic Matter Interference:** Most disinfectants (like alcohols and halogens) are neutralized by organic matter; however, **Phenolics (Cresol)** maintain efficacy, making them ideal for excreta. * **Bleaching Powder:** Used for disinfecting water and spillages, but requires a high concentration for faeces due to organic load.

Q: Which of the following provides the best guarantee for sterilization in a heat sterilizer?

Using a bacterial spore test. **Explanation:** The gold standard for monitoring sterilization is the use of **Biological Indicators (BIs)**, specifically bacterial spore tests. This is because bacterial spores are the most heat-resistant living organisms; if the sterilization process is potent enough to kill these highly resistant spores, it is logically inferred that all other pathogenic microorganisms (bacteria, viruses, fungi) have also been destroyed. * **Why Option B is Correct:** Biological indicators provide the only direct evidence of microbial kill. For autoclaves (moist heat), **_Geobacillus stearothermophilus_** spores are used because they are highly thermophilic. For dry heat (hot air oven), **_Bacillus atrophaeus_** (formerly *B. subtilis var. niger*) is used. * **Why Option A is Incorrect:** Temperature and pressure gauges are **Physical/Mechanical Indicators**. They only measure the conditions inside the chamber at a specific moment but do not account for "cold spots" or air pockets inside the packs. * **Why Option D is Incorrect:** Chemical indicators (e.g., Bowie-Dick tape) only confirm that a certain temperature was reached. They do not guarantee that the temperature was maintained for the required "holding time" necessary for sterilization. * **Why Option C is Incorrect:** While HBV is resistant, it is significantly less resistant than bacterial spores. Testing against HBV is neither practical nor as stringent as a spore test. **High-Yield NEET-PG Pearls:** * **Autoclave:** 121°C at 15 psi for 15–20 mins. BI: *Geobacillus stearothermophilus*. * **Hot Air Oven:** 160°C for 2 hours. BI: *Bacillus atrophaeus*. * **Ethylene Oxide (ETO):** BI: *Bacillus atrophaeus*. * **Ionizing Radiation:** BI: *Bacillus pumilus*.

Q: Which of the following statements about alcohol in disinfection is FALSE?

Alcohol has sporicidal activity.. **Explanation:** The correct answer is **C (Alcohol has sporicidal activity)** because alcohols are intermediate-level disinfectants that lack the ability to penetrate the thick, protective protein coats of bacterial spores. **1. Why Option C is False (The Correct Answer):** Alcohols (Ethanol and Isopropyl alcohol) act by denaturing proteins and dissolving lipid membranes. While they are highly effective against vegetative bacteria, fungi, and enveloped viruses, they are **not sporicidal**. They also lack significant activity against non-enveloped viruses and fungal spores. To kill bacterial spores, "high-level" disinfectants or sterilants like Glutaraldehyde or Autoclaving are required. **2. Why the other options are True:** * **Options A & B:** Both **Ethyl alcohol (Ethanol)** and **Isopropyl alcohol (Isopropanol)** are the primary alcohols used in clinical settings. Isopropyl alcohol is slightly more bactericidal and less volatile than ethanol. * **Option D:** Alcohol is rapidly **bactericidal** against vegetative forms of Gram-positive and Gram-negative bacteria (e.g., *S. aureus*, *E. coli*, and *M. tuberculosis*). **High-Yield NEET-PG Clinical Pearls:** * **Optimal Concentration:** Alcohols are most effective at a concentration of **60%–90%** in water. Absolute alcohol (100%) is less effective because protein denaturation requires the presence of water. * **Contact Time:** They are "flash" disinfectants; they act quickly but have no residual activity due to rapid evaporation. * **Clinical Use:** Primarily used as skin antiseptics (spirit) and for disinfecting small surfaces like stethoscope diaphragms and thermometer ports. * **Methyl Alcohol:** Not used for disinfection as it is toxic and has relatively weak bactericidal action.

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: 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.

Question 1

All of the following are used in the disinfection of faeces, EXCEPT:

Accepted Answer

Glutaraldehyde

Answer

Cresol

Answer

Formalin

Answer

Bleaching powder

Question 2

Which of the following is used commercially for sterilization of disposable plastic items?

Accepted Answer

Ethyl alcohol

Answer

Autoclave

Answer

Glutaraldehyde

Answer

Ethylene oxide

Question 3

Which of the following provides the best guarantee for sterilization in a heat sterilizer?

Accepted Answer

Using a bacterial spore test

Answer

Recording temperature and pressure readings from the sterilizer gauge

Answer

Determining the ability of a sterilizer to kill the hepatitis B virus

Answer

Using a chemical indicator strip or pouch

Question 4

Which of the following statements about alcohol in disinfection is FALSE?

Accepted Answer

Alcohol has sporicidal activity.

Answer

Ethanol is used.

Answer

Isopropyl alcohol is used.

Answer

Alcohol has bactericidal activity.

Question 5

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 6

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 7

Irradiation can be used to sterilize all except:

Accepted Answer

Bronchoscope

Answer

Bone graft

Answer

Suture

Answer

Artificial tissue graft

Question 8

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

Accepted Answer

2% glutaraldehyde

Answer

2% formalin

Answer

Autoclaving

Answer

Boiling

Question 9

Which of the following is an intermediate level disinfectant?

Accepted Answer

Iodophores

Answer

Glutaraldehyde

Answer

Quaternary ammonium compound

Answer

Hydrogen peroxide

Question 10

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

Sterilization and Disinfection — MCQs

Sterilization and Disinfection — MCQs

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