Sterilization and Disinfection — MCQs

Sterilization and Disinfection Indian Medical PG Practice Questions and MCQs

Question 111: Chlorine exerts a disinfectant action in all except:

A. Cetrimide (Correct Answer)
B. Bleaching Powder
C. Halozone tablets
D. Sodium hypochlorite

Explanation: ### Explanation The disinfectant action of chlorine is based on the release of **free chlorine** and the formation of **hypochlorous acid (HOCl)** when added to water. Hypochlorous acid is a powerful oxidizing agent that inhibits bacterial enzymes and damages cell membranes. **Why Cetrimide is the correct answer:** **Cetrimide** is a **Quaternary Ammonium Compound (QAC)**, which is a type of cationic surface-active agent (detergent). It works by disrupting the cell membrane and denaturing proteins. It does **not** contain chlorine and does not exert its action through chlorination or oxidation via hypochlorous acid. Therefore, it is the odd one out in this list. **Analysis of Incorrect Options:** * **Bleaching Powder (Calcium hypochlorite):** This is a common source of chlorine used for disinfecting large bodies of water and floors. It releases chlorine upon contact with water. * **Halozone tablets:** These are organic chloramines used specifically for the disinfection of small quantities of drinking water (e.g., for travelers or soldiers). * **Sodium hypochlorite:** A widely used liquid disinfectant (e.g., household bleach). It is the gold standard for disinfecting surfaces contaminated with blood-borne pathogens like HIV and Hepatitis B. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Chlorine’s efficacy is pH-dependent; it is most effective at an acidic or neutral pH because hypochlorous acid dissociates at high pH. * **Organic Matter:** Chlorine is easily neutralized by organic matter (pus, blood, feces). Surfaces must be cleaned before application. * **HIV/HBV Protocol:** Sodium hypochlorite (1% for small spills, 10% for large spills) is the disinfectant of choice for blood spills. * **Cetrimide + Chlorhexidine:** This combination is known as **Savlon**, frequently used for wound cleansing and preoperative skin preparation.

Question 112: Disposable syringes are sterilized by which method?

A. 2% glutaraldehyde
B. Autoclave
C. Hot air oven
D. Ionizing radiation (Correct Answer)

Explanation: ### Explanation **1. Why Ionizing Radiation is Correct:** Disposable syringes are typically made of heat-sensitive plastics (like polypropylene). **Ionizing radiation**, specifically **Gamma rays** (from Cobalt-60) or **Electron beams**, is the method of choice for sterilizing such pre-packed, heat-sensitive medical devices. This is often referred to as **"Cold Sterilization"** because it achieves sterilization without a significant rise in temperature. It has high penetrating power, allowing it to sterilize items already sealed in their final plastic packaging. **2. Why the Other Options are Incorrect:** * **2% Glutaraldehyde (Cidex):** This is a high-level disinfectant used primarily for heat-sensitive semi-critical items like **endoscopes and bronchoscopes**. It is not used for mass-scale sterilization of disposable syringes. * **Autoclave (Moist Heat):** While highly effective for surgical instruments, the high temperature ($121^\circ\text{C}$) and pressure can melt or deform the plastic components of disposable syringes. * **Hot Air Oven (Dry Heat):** This requires very high temperatures ($160^\circ\text{C}$ for 1 hour), which would destroy plastic syringes. It is reserved for glassware, forceps, and oily substances. **3. High-Yield Clinical Pearls for NEET-PG:** * **Ethylene Oxide (EtO):** If ionizing radiation is not an option, EtO is the alternative for heat-sensitive equipment (e.g., heart-lung machines, respirators). * **Gamma Radiation Symbol:** Look for the "Red Dot" or "Magenta" indicator on packaging, which confirms exposure to radiation. * **Bacillus pumilus:** This is the biological indicator used to test the efficacy of ionizing radiation. * **Disposable items** (syringes, catheters, sutures) are the most common items sterilized by this method.

Question 113: Sterilization of liquid paraffin is done by:

A. Autoclaving
B. Hot air oven (Correct Answer)
C. UV radiation
D. Inspissation

Explanation: **Explanation:** The correct answer is **Hot air oven (B)**. Liquid paraffin, along with other oils, fats, waxes, and powders (like talcum powder), are **non-aqueous substances**. These materials are impermeable to moisture. Since the efficacy of autoclaving depends on the penetration of saturated steam, it is ineffective for sterilizing oily liquids. **Hot air oven** utilizes **dry heat**, which kills microorganisms by causing oxidative damage to cell components and denaturation of proteins. It is the method of choice for substances that are either damaged by moisture or cannot be reached by steam. For liquid paraffin, the standard cycle is **160°C for 1 hour**. **Why other options are incorrect:** * **Autoclaving (Moist Heat):** Steam cannot penetrate oils or powders. Water and oil do not mix; therefore, the steam only touches the surface, leaving the interior unsterilized. * **UV Radiation:** This is a form of non-ionizing radiation used primarily for surface disinfection and air sterilization in OTs. It has very poor penetrating power and cannot sterilize the bulk of a liquid. * **Inspissation:** This involves heating at 80-85°C for 30 minutes on three successive days. It is specifically used for sterilizing media containing high protein (e.g., LJ medium, Loeffler’s serum slope) to prevent coagulation. **High-Yield Clinical Pearls for NEET-PG:** * **Glassware** (syringes, petri dishes, flasks) is also sterilized in a Hot Air Oven. * **Sharp instruments** (scalpels, scissors) are ideally sterilized in a Hot Air Oven to prevent the dulling of edges caused by moisture in autoclaves. * **Biological Indicator** for Hot Air Oven: *Bacillus atrophaeus* (formerly *B. subtilis* var. *niger*). * **Biological Indicator** for Autoclave: *Geobacillus stearothermophilus*.

Question 114: Prions are best killed by which of the following methods?

A. Autoclaving at 121°C
B. 5% formalin
C. Sodium hydroxide (Correct Answer)
D. Sodium hypochlorite

Explanation: **Explanation:** Prions are proteinaceous infectious particles that lack nucleic acids and are notoriously resistant to standard sterilization methods. They are more resistant than bacterial spores, which are typically the benchmark for sterilization. **Why Sodium Hydroxide (NaOH) is correct:** Prions are inactivated by chemicals that denature proteins or disrupt their complex folding. **1N Sodium Hydroxide (NaOH)** for 1 hour is a recommended chemical method for prion decontamination. It effectively hydrolyzes the peptide bonds of the scrapie-associated prion protein (PrPSc), rendering it non-infectious. **Analysis of Incorrect Options:** * **Autoclaving at 121°C:** Standard autoclaving (121°C for 15-30 mins) is insufficient for prions. Prions require higher temperatures, specifically **134°C for 1-1.5 hours** (Gravity displacement) or 18 minutes (Pre-vacuum) to ensure total inactivation. * **5% Formalin:** Formaldehyde and glutaraldehyde are actually **contraindicated** for prions. These are "fixing" agents that stabilize the protein structure, potentially making the prions even more resistant to subsequent heat sterilization. * **Sodium Hypochlorite:** While 5.25% sodium hypochlorite (bleach) is effective against prions, **Sodium Hydroxide** is traditionally considered the gold standard chemical disinfectant in medical literature and competitive exams for complete protein degradation. **High-Yield Clinical Pearls for NEET-PG:** * **Hierarchy of Resistance:** Prions > Bacterial Spores (B. stearothermophilus) > Mycobacteria > Non-enveloped viruses > Fungi > Vegetative bacteria > Enveloped viruses (HIV/HBV). * **Best Physical Method:** Autoclaving at 134°C for 1 hour. * **Best Chemical Method:** 1N NaOH or 5.25% Sodium Hypochlorite for 1 hour. * **Incineration:** This is the most effective way to dispose of contaminated disposable materials.

Question 115: What is the best disinfectant for cholera stools?

A. Bleaching powder
B. Coal-tar
C. Cresol (Correct Answer)
D. Formalin

Explanation: **Explanation:** The correct answer is **Cresol** (Option C). **Why Cresol is the correct answer:** In the management of infectious diseases like cholera, the disinfection of excreta (stools and vomit) is critical. **Cresol (Liquor Saponis Antisepticus/Lysol)** is considered the disinfectant of choice for cholera stools because it is highly effective against *Vibrio cholerae* and maintains its efficacy even in the presence of high organic matter. For effective disinfection, a 5% cresol solution should be used with a contact time of at least 1–2 hours. **Analysis of Incorrect Options:** * **Bleaching powder (A):** While it is the gold standard for disinfecting water (chlorination), it is less effective for stools because organic matter rapidly neutralizes free chlorine, rendering it ineffective unless used in very high concentrations. * **Coal-tar (B):** This is a crude source of phenolic compounds but is not used in standardized clinical practice for stool disinfection compared to its refined derivative, cresol. * **Formalin (D):** Formalin is primarily used for preserving tissues (histopathology) or fumigation (as a gas). It is not practical or standard for the routine disinfection of large volumes of liquid excreta due to its pungent odor and irritant properties. **High-Yield Clinical Pearls for NEET-PG:** * **Cholera Stool Disinfection:** Cresol (5%) or Bleaching powder (if cresol is unavailable, but requires longer contact). * **Sputum Disinfection:** Burning (best) or Cresol/Autoclaving. * **HIV/Hepatitis B Spills:** Sodium Hypochlorite (1% for small spills, 10% for large blood spills). * **Cystoscope/Endoscope:** 2% Glutaraldehyde (Cidex) is the agent of choice. * **Vibrio cholerae** is highly sensitive to acids but survives well in alkaline media (pH 8.5–9.5).

Question 116: Which of the following methods is most commonly used for sterilizing syringes and glassware?

A. Irradiation
B. Autoclave
C. Hot air oven (Correct Answer)
D. Glutaraldehyde

Explanation: **Explanation:** The **Hot Air Oven** is the method of choice for sterilizing **glassware** (like syringes, petri dishes, and pipettes) and anhydrous materials (like powders and oils). It utilizes **dry heat**, which kills microorganisms primarily through the **oxidation** of intracellular components and protein denaturation. Dry heat is preferred for glassware because it does not leave moisture or condensation, preventing the "clouding" of glass and ensuring that items remain dry and ready for immediate use. **Analysis of Incorrect Options:** * **Irradiation (Option A):** Also known as "Cold Sterilization," it is primarily used for heat-sensitive, pre-packed disposable items like plastic syringes, catheters, and sutures. It is not the standard for reusable laboratory glassware. * **Autoclave (Option B):** Uses **moist heat** (steam under pressure). While highly effective, it can lead to moisture entrapment in narrow-necked glassware and may cause "blooming" or erosion of certain types of glass over time. It is the gold standard for culture media and surgical dressings. * **Glutaraldehyde (Option D):** A high-level disinfectant (2% solution, e.g., Cidex) used for heat-sensitive endoscopes and bronchoscopes. It is a chemical disinfectant, not a physical sterilization method for routine glassware. **High-Yield Clinical Pearls for NEET-PG:** * **Standard Settings:** Hot air oven requires **160°C for 2 hours** or **170°C for 1 hour**. * **Sterilization Control:** The biological indicator used for the Hot Air Oven is ***Bacillus subtilis*** (var. *niger*). * **Contraindication:** Do not use dry heat for surgical dressings, rubber items, or volatile liquids as they may burn or degrade. * **Mechanism:** Dry heat = Oxidation; Moist heat = Protein coagulation.

Question 117: What is inspissation?

A. Heating at 160°C for 15 minutes
B. Heating at 160°C for 30 seconds
C. Heating at 80°C for 30 minutes (Correct Answer)
D. Heating at 120°C for 15 seconds

Explanation: **Explanation** **Inspissation** is a specialized method of sterilization used for culture media containing high amounts of protein, such as egg or serum, which would coagulate and be ruined by the high temperatures of an autoclave. 1. **Why Option C is Correct:** Inspissation involves heating the medium at **80°C to 85°C for 30 minutes** on three successive days. This process is carried out in an instrument called an **Inspissator**. The fractional heating allows for the sterilization of heat-sensitive media like **Lowenstein-Jensen (LJ) medium** (used for *M. tuberculosis*) and **Loeffler’s Serum Slope** (used for *C. diphtheriae*). 2. **Why Other Options are Incorrect:** * **Option A (160°C for 15-60 mins):** This describes the standard cycle for a **Hot Air Oven**, used for dry heat sterilization of glassware and forceps. * **Option B (160°C for 30 seconds):** This does not correspond to any standard sterilization protocol; it is too short for dry heat and too high for moist heat. * **Option D (120°C for 15 seconds):** This resembles "Flash Pasteurization" (72°C for 15s) or Ultra-High Temperature (UHT) processing, but the temperature/time combination provided is not a standard medical sterilization benchmark. **High-Yield Clinical Pearls for NEET-PG:** * **Principle:** Inspissation works by the coagulation of proteins. * **Key Media:** Always associate Inspissation with **LJ Medium** and **Loeffler’s Serum Slope**. * **Tyndallization vs. Inspissation:** Tyndallization uses 100°C for 20 minutes (3 days) for sugar-containing media, whereas Inspissation uses lower temperatures (80-85°C) for protein-rich media. * **Vaccine Sterilization:** Most vaccines are sterilized at much lower temperatures (e.g., 56°C for 1 hour) in a water bath.

Question 118: What is the most effective method of sterilization by heat?

A. Hot air
B. Boiling
C. Steam
D. Steam under pressure (Correct Answer)

Explanation: **Explanation:** The correct answer is **Steam under pressure (Autoclaving)**. This is considered the most effective method of sterilization because it utilizes **moist heat** at temperatures above $100^\circ\text{C}$. **Why it is correct:** The underlying mechanism is the **denaturation and coagulation of bacterial proteins**. Steam under pressure has high penetrating power. When steam condenses on a cooler surface, it releases a large amount of latent heat ($518 \text{ calories/gram}$), which rapidly kills even the most resistant bacterial spores. The standard parameters are $121^\circ\text{C}$ at $15 \text{ psi}$ for $15\text{--}20$ minutes. **Why other options are incorrect:** * **Hot air (Option A):** This is a form of **dry heat**. It is less effective than moist heat because dry heat kills by oxidative damage, which requires higher temperatures ($160^\circ\text{C}$) and longer durations ($2$ hours) to achieve the same level of sterility. * **Boiling (Option B):** Boiling at $100^\circ\text{C}$ is a method of **disinfection**, not sterilization. While it kills most vegetative forms, it cannot reliably kill highly resistant endospores (e.g., *Clostridium tetani*). * **Steam (Option C):** Free steam at $100^\circ\text{C}$ (Tyndallization) is used for heat-sensitive media but is less efficient and more time-consuming than pressurized steam, as it does not reach the temperatures required to kill spores in a single cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Sterilization Indicator:** The biological indicator used to check the efficacy of an autoclave is **_Geobacillus stearothermophilus_** (formerly *Bacillus stearothermophilus*). * **Flash Sterilization:** Done at $134^\circ\text{C}$ for $3$ minutes for urgent surgical instruments. * **Sharp Instruments:** These are preferably sterilized by **Hot Air Oven** to prevent the dulling of blades that occurs with moist heat.

Question 119: 40% formalin is used for sterilization of which of the following?

A. Plastic syringes
B. All microbes and spores (Correct Answer)
C. Clothes
D. Stitches

Explanation: **Explanation:** **Formaldehyde (Formalin)** is a high-level disinfectant and a potent alkylating agent. It works by alkylating amino, carboxyl, and hydroxyl groups in nucleic acids and proteins, effectively killing all forms of microbial life. **1. Why the Correct Answer is Right:** * **Broad Spectrum:** 40% Formaldehyde (commercially known as 100% Formalin) is a powerful **sporicidal** agent. It is effective against vegetative bacteria, fungi, viruses (including HBV and HIV), and highly resistant bacterial spores. * **Mechanism:** It causes irreversible cross-linking of proteins and DNA, leading to cell death. Because it destroys spores, it is classified as a chemical sterilant rather than just a disinfectant. **2. Why Other Options are Incorrect:** * **A. Plastic Syringes:** These are typically sterilized using **Ethylene Oxide (EtO)** or **Gamma radiation**. Formalin is not used due to its pungent odor and potential toxicity/residue on medical devices. * **C. Clothes:** Formalin is generally avoided for clothing because it is an irritant to the skin and respiratory tract. Clothes are usually sterilized via **Autoclaving** or disinfected using laundry chemicals. * **D. Stitches (Sutures):** Absorbable sutures (like Catgut) are sterilized using **Ionizing Radiation (Gamma rays)**. Non-absorbable sutures may be autoclaved or pre-packed using EtO. **Clinical Pearls for NEET-PG:** * **Fumigation:** Formaldehyde gas (generated from formalin) is the gold standard for sterilizing **Operation Theaters (OT)** and biological safety cabinets. * **Concentration:** 10% Neutral Buffered Formalin is the standard concentration used for **histopathology** specimen preservation. * **Toxicity:** It is a known carcinogen and a potent mucosal irritant; hence, areas must be neutralized (usually with ammonia) after fumigation.

Question 120: Sputum can be disinfected by which of the following methods?

A. Boiling
B. Burning
C. 5% cresol (Correct Answer)
D. Autoclaving

Explanation: ### Explanation The correct answer is **5% cresol**. **1. Why 5% Cresol is Correct:** Sputum is a highly viscous, proteinaceous substance that often contains high loads of *Mycobacterium tuberculosis*. For routine bedside disinfection, **phenolic compounds** like 5% cresol (or Lysol) are the agents of choice. They act by disrupting cell membranes and precipitating proteins. Unlike many other disinfectants, cresols remain active in the presence of organic matter (mucus/protein), making them ideal for decontaminating sputum cups before final disposal. **2. Analysis of Incorrect Options:** * **Boiling:** While boiling kills most vegetative bacteria, it is not a reliable method for sputum because the thick mucus can protect organisms in the center of the mass. Furthermore, it is impractical for large-scale bedside waste management. * **Burning (Incineration):** This is the method of choice for the **final disposal** of sputum (especially if collected in paper cups). However, the question asks for a method to *disinfect* it. In a clinical setting, sputum is typically disinfected chemically before disposal. * **Autoclaving:** This is the gold standard for sterilization (121°C for 15 mins). While it would effectively kill all organisms in sputum, it is generally reserved for laboratory waste or reusable equipment rather than routine bedside disinfection of patient excreta. **3. High-Yield Clinical Pearls for NEET-PG:** * **Sputum Disposal:** The best method for final disposal of sputum is **Incineration**. * **Sodium Hypochlorite (1%):** Used for blood spills and HIV/HBV contaminated surfaces, but less effective for thick sputum compared to phenols. * **Pre-treatment:** Sputum should be kept in contact with 5% cresol for at least **1 hour** for effective disinfection. * **Phenol Coefficient:** This is a measure of the killing efficacy of a disinfectant compared to phenol; cresols have a higher coefficient than pure phenol.

Practice by Chapter

Physical Methods of Sterilization

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Chemical Methods of Sterilization

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Disinfectants and Antiseptics

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Sterilization Monitoring

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Hospital Sterilization Protocols

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Surgical Instruments Sterilization

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Endoscope Reprocessing

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Biological Indicators

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Factors Affecting Sterilization

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Quality Control in Sterilization

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Biofilm and Its Implications

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Regulatory Aspects of Sterilization

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