Sterilization and Disinfection — MCQs

Sterilization and Disinfection Indian Medical PG Practice Questions and MCQs

Question 61: The active disinfecting property of bleaching powder is mainly due to the formation of which substance?

A. Hydrogen chloride
B. Hypochloric acid
C. Hypochlorous acid (Correct Answer)
D. Chloramines

Explanation: ### Explanation **1. Why Hypochlorous Acid (HOCl) is Correct:** Bleaching powder (Calcium hypochlorite) is a chlorine-releasing agent. When dissolved in water, it undergoes a chemical reaction to produce **Hypochlorous acid (HOCl)**. * **Mechanism:** HOCl is a neutral molecule that easily penetrates the negatively charged cell walls of microorganisms. Once inside, it acts as a powerful oxidizing agent, denaturing bacterial proteins and inactivating essential enzymes. * **pH Dependency:** The efficacy of chlorine disinfection is highly dependent on pH. At a lower pH (acidic to neutral), HOCl is the dominant species, making the disinfection process most effective. **2. Why Other Options are Incorrect:** * **Hydrogen chloride (HCl):** This is a strong acid that does not possess the specific oxidative properties required for effective microbial disinfection at standard concentrations. * **Hypochloric acid:** This is a chemical misnomer and does not exist as a functional disinfecting agent in this context. * **Chloramines:** These are formed when chlorine reacts with ammonia or organic nitrogen. While they do have some disinfectant properties (used in municipal water treatment), they are significantly slower and less potent than HOCl. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Chlorine Demand:** This refers to the amount of chlorine consumed by organic/inorganic matter in water before a free residual chlorine level is established. * **Free Residual Chlorine:** For effective disinfection of drinking water, the recommended level is **0.5 mg/L** with a contact time of at least **30 minutes**. * **Orthotolidine Test (OT):** Used to detect both free and combined chlorine. The **Orthotolidine Arsenite (OTA) Test** is specifically used to distinguish between free and combined chlorine. * **Disinfection of Spills:** For blood spills (e.g., HIV, HBV), **1% Sodium Hypochlorite** (10,000 ppm) is the standard recommendation.

Question 62: What is true about the Chick-Martin test?

A. Phenol is taken as standard.
B. It is a test for the efficacy of a disinfectant.
C. It assesses disinfection in the presence of organic matter.
D. All of the above. (Correct Answer)

Explanation: The **Chick-Martin Test** is a standardized laboratory method used to evaluate the efficacy of disinfectants, specifically designed to overcome the limitations of the Rideal-Walker test. ### **Explanation of Options:** * **A. Phenol is taken as standard:** Like the Rideal-Walker test, the Chick-Martin test uses **Pure Phenol** as the reference standard. The efficacy of the test disinfectant is compared against phenol to determine its relative potency. * **B. Efficacy of a disinfectant:** It is a modification of the "Phenol Coefficient" test. It determines the killing power of a disinfectant against a specific test organism (usually *Salmonella typhi* or *Staphylococcus aureus*). * **C. Presence of organic matter:** This is the **defining characteristic** of the Chick-Martin test. While the Rideal-Walker test measures efficacy in distilled water, the Chick-Martin test adds **3% dried yeast** (or sometimes feces) to simulate real-world conditions where organic matter often neutralizes disinfectants. Since all three statements accurately describe the parameters and purpose of the test, **Option D is correct.** ### **High-Yield Clinical Pearls for NEET-PG:** 1. **Rideal-Walker vs. Chick-Martin:** * **Rideal-Walker:** No organic matter used. It may overstate the effectiveness of a disinfectant. * **Chick-Martin:** Uses organic matter (yeast). It provides a more realistic "in-use" assessment. 2. **Kelsey-Sykes Test:** Another high-yield test that assesses the "capacity" of a disinfectant to handle subsequent loads of organisms (Capacity Test). 3. **In-Use Test (Maurer’s Test):** Used to check if the disinfectant solution currently being used in hospital wards/theaters is contaminated or has lost its potency.

Question 63: What is the best method for sterilization of sera?

A. Filtration (Correct Answer)
B. Autoclaving
C. Radiation
D. Heating

Explanation: **Explanation:** The correct answer is **Filtration**. **Why Filtration is the best method:** Sera (serum), along with other biological fluids like toxins, vaccines, and antibiotic solutions, are **heat-sensitive (thermolabile)**. These substances contain proteins that undergo denaturation and lose their biological activity if exposed to high temperatures. Filtration is a "cold sterilization" technique that physically removes bacteria and fungi by passing the liquid through filters with pore sizes typically around **0.22 μm**. This ensures the serum remains sterile without compromising its chemical integrity. **Why other options are incorrect:** * **Autoclaving (B):** This uses moist heat at 121°C. Such high temperatures would coagulate the proteins in the serum, making it useless. * **Radiation (C):** While used for disposable plastics (gamma rays), ionizing radiation can alter the molecular structure of complex biological proteins in serum. * **Heating (D):** Even low-level heating (like inspissation or pasteurization) can lead to partial denaturation and is generally insufficient to achieve absolute sterility for serum used in sensitive laboratory cultures. **High-Yield Clinical Pearls for NEET-PG:** * **Types of Filters:** Historically, Seitz (asbestos) and Chamberland (porcelain) filters were used. Modern labs use **Membrane filters** (Cellulose acetate/nitrate). * **Air Filtration:** HEPA filters are used in biological safety cabinets and OTs to remove particles >0.3 μm. * **Sterilization of Culture Media:** Most media are autoclaved, but those containing egg or serum (e.g., **LJ Medium, Loeffler’s Serum Slope**) are sterilized via **Inspissation** (heating at 80-85°C for 30 mins on three consecutive days).

Question 64: Which method is used for sterilization of endoscopes?

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

Explanation: **Explanation:** **1. Why Glutaraldehyde is the Correct Answer:** Endoscopes (such as gastroscopes and bronchoscopes) are classified as **semi-critical items** because they come into contact with mucous membranes but do not penetrate sterile tissue. Most endoscopes are **thermolabile** (heat-sensitive), meaning they would be damaged by the high temperatures of steam or dry heat sterilization. **2% Glutaraldehyde (Cidex)** is the gold standard for high-level disinfection/sterilization of these instruments. It acts by alkylation of amino, carboxyl, and hydroxyl groups, effectively killing bacteria, spores, fungi, and viruses. A contact time of 20 minutes is required for disinfection, while 10 hours is needed for true sterilization (sporicidal action). **2. Why Other Options are Incorrect:** * **B. Autoclave:** Uses moist heat (121°C). While it is the most effective method for surgical instruments, the high pressure and temperature would melt the plastic components and damage the delicate fiber-optics of most endoscopes. * **C. Formaldehyde:** Though a potent disinfectant, it is rarely used for endoscopes due to its pungent odor, slow action, and potential carcinogenicity. It is primarily used for fumigating operation theaters or preserving tissues. * **D. Hot Air Oven:** Uses dry heat (160°C). This is entirely unsuitable for any heat-sensitive equipment and is reserved for glassware, forceps, and oils. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cidex Shelf Life:** Once activated by adding an alkalizing agent, the solution remains active for **14 days**. * **Ortho-phthalaldehyde (OPA):** A newer alternative to glutaraldehyde that is more stable, faster-acting, and does not require activation, though it is more expensive. * **Steris System:** Uses **Peracetic acid** for rapid cold sterilization of endoscopes. * **Prions:** Glutaraldehyde is ineffective against prions; in fact, it may "fix" them to the instrument.

Question 65: Sputum can be disinfected by all except?

A. Autoclaving
B. Chlorhexidine (Correct Answer)
C. Boiling
D. Cresol

Explanation: **Explanation:** The correct answer is **Chlorhexidine**. Sputum is a biological fluid that often contains high concentrations of organic matter and potentially resistant pathogens like *Mycobacterium tuberculosis*. To effectively disinfect sputum, an agent must be able to penetrate organic debris and possess **tuberculocidal** activity. **Why Chlorhexidine is the correct answer:** Chlorhexidine is a biguanide antiseptic primarily used for skin disinfection and preoperative scrubbing. It is **ineffective** against sputum because: 1. It is **not tuberculocidal** (it is primarily bacteriostatic/bactericidal against vegetative Gram-positive bacteria). 2. It is **inactivated by organic matter** (pus, blood, and mucus), which are abundant in sputum. **Analysis of other options:** * **Autoclaving (Option A):** This is the gold standard for sterilization. It uses moist heat under pressure (121°C for 15 mins) to kill all microbial life, including highly resistant spores and Mycobacteria. It is the preferred method for laboratory disposal of sputum. * **Boiling (Option C):** Boiling for 20–30 minutes is a common method for disinfecting sputum in resource-limited settings. It effectively kills most vegetative pathogens, including *M. tuberculosis*. * **Cresol/Lysol (Option D):** Phenolic compounds like Cresol (5%) are highly effective for sputum disinfection. Unlike Chlorhexidine, phenols are **not easily inactivated by organic matter** and are potent tuberculocidal agents. **NEET-PG High-Yield Pearls:** * **Best method for sputum disposal:** Burning (Incineration) or Autoclaving. * **Chemical of choice for sputum:** 5% Cresol or 1% Sodium Hypochlorite (if organic load is low). * **Chlorhexidine (Savlon/Dettol components):** Excellent for skin but poor against *Pseudomonas*, *M. tuberculosis*, and bacterial spores. * **Pre-cleaning:** Always remember that organic matter protects microbes; hence, physical cleaning is often required before chemical disinfection.

Question 66: A Seitz filter is a type of?

A. Candle filter
B. Asbestos filter (Correct Answer)
C. Membrane filter
D. Sintered glass filter

Explanation: **Explanation:** The **Seitz filter** is a classic example of an **Asbestos filter**. It consists of a disc made of asbestos material (often magnesium silicate) supported by a metal perforated plate. It works primarily through the mechanism of **adsorption**, where microorganisms are trapped within the fibrous matrix of the asbestos disc. While historically significant for sterilizing heat-sensitive liquids like sera and sugars, its use has declined due to the carcinogenic nature of asbestos and the tendency of the filter to alkaline the filtrate. **Analysis of Options:** * **A. Candle filter:** These are made of hollowed-out porcelain or infusorial earth (e.g., Berkefeld, Chamberland filters). They act as "sieves" and are commonly used for water purification. * **C. Membrane filter:** These are modern filters made of cellulose esters (acetate or nitrate). They are the current gold standard for sterilizing fluids and air (HEPA) because they have a uniform pore size (typically 0.22 µm) and do not absorb the fluid being filtered. * **D. Sintered glass filter:** These are made by fusing finely powdered glass. They are used for filtering corrosive chemicals and do not shed fibers into the filtrate, unlike asbestos. **High-Yield Facts for NEET-PG:** * **Mechanism of Filtration:** Most filters work by a combination of **sieving** (mechanical) and **adsorption** (electrostatic). * **Filter pore size for sterilization:** 0.22 µm is required to remove most bacteria (except *Mycoplasma* and viruses). * **Air Sterilization:** Laminar air flow benches use **HEPA filters**, which remove particles >0.3 µm with 99.97% efficiency. * **Testing Filter Integrity:** The **Bubble Point Test** is used to ensure a filter is not damaged.

Question 67: Pasteurization of milk does not kill which of the following organisms?

A. Bacillus anthracis (Correct Answer)
B. Brucella species
C. Mycobacterium tuberculosis
D. Streptococcus species

Explanation: **Explanation:** The primary objective of pasteurization is to eliminate common non-spore-forming pathogenic bacteria found in milk, making it safe for human consumption without significantly altering its nutritional value. **1. Why Bacillus anthracis is the correct answer:** *Bacillus anthracis* is a **spore-forming** bacterium. Pasteurization (Holder method: 63°C for 30 mins; Flash method: 72°C for 15-20 secs) is a form of disinfection, not sterilization. While it kills vegetative cells, it is **ineffective against bacterial spores**, which are highly resistant to heat. To kill *B. anthracis* spores, higher temperatures (e.g., autoclaving at 121°C) are required. **2. Analysis of Incorrect Options:** * **Mycobacterium tuberculosis:** Historically, this was the index organism for determining pasteurization temperatures because it is the most heat-resistant non-spore-forming pathogen. Pasteurization effectively kills it. * **Brucella species:** These are common milk-borne pathogens causing Brucellosis (undulant fever). They are highly heat-sensitive and are destroyed during the process. * **Streptococcus species:** These are vegetative organisms that can cause bovine mastitis or human sore throat via contaminated milk; they are easily neutralized by pasteurization. **Clinical Pearls for NEET-PG:** * **Index Organism:** Currently, ***Coxiella burnetii*** (the agent of Q fever) is considered the most heat-resistant non-spore-forming pathogen in milk and is used as the standard for modern pasteurization. * **Phosphatase Test:** Used to check the efficacy of pasteurization. If the enzyme phosphatase is inactivated, the process is considered successful. * **Sterilization vs. Disinfection:** Remember that pasteurization **disinfects** (kills pathogens) but does not **sterilize** (does not kill all microbes or spores).

Question 68: Which agent is used for sterilizing endoscopy tubes?

A. 2% glutaraldehyde (Correct Answer)
B. Sodium hypochlorite
C. Ethylene oxide
D. Ionizing radiation

Explanation: **Explanation:** **1. Why 2% Glutaraldehyde is correct:** Glutaraldehyde (commonly known by the brand name **Cidex**) is the agent of choice for "cold sterilization" of heat-sensitive delicate instruments like endoscopes, cystoscopes, and bronchoscopes. It is a high-level disinfectant that acts by alkylating amino, carboxyl, and hydroxyl groups of proteins. A **2% solution** requires an immersion time of **20 minutes** for high-level disinfection and **10 hours** for absolute sterilization (killing spores). It is preferred because it is non-corrosive to metals, rubber, and lenses. **2. Why other options are incorrect:** * **B. Sodium hypochlorite:** This is a strong oxidizing agent used primarily for disinfecting surfaces, blood spills (1% concentration), and HIV-contaminated waste. It is highly corrosive to metals and can damage the delicate components of an endoscope. * **C. Ethylene oxide (ETO):** While ETO is used for heat-sensitive items (like heart-lung machines or catheters), it is a gas sterilization method that requires long aeration times to remove toxic residues. Glutaraldehyde is more practical and faster for the rapid turnover of endoscopes in clinical settings. * **D. Ionizing radiation:** Also known as "cold sterilization," this is used for mass sterilization of disposable items (syringes, sutures, catheters) at a commercial level. It is not feasible for hospital-based reprocessing of reusable endoscopes. **Clinical Pearls for NEET-PG:** * **Ortho-phthalaldehyde (OPA):** A newer alternative to glutaraldehyde; it is more stable, faster-acting (5–12 mins), and does not require activation, though it is more expensive. * **Sterilization Monitoring:** The efficacy of glutaraldehyde is checked using **test strips** to ensure the concentration remains above the Minimum Effective Concentration (MEC), usually 1.5%. * **Safety:** Glutaraldehyde fumes are toxic; instruments must be thoroughly rinsed with sterile water after immersion to prevent "chemical colitis" in patients.

Question 69: Which of the following is true about the Holder method of pasteurization?

A. It kills all bacteria and spores.
B. It kills all bacteria except Coxiella burnetti. (Correct Answer)
C. It kills 95% of microorganisms.
D. All bacteria are destroyed.

Explanation: **Explanation:** **1. Why Option B is Correct:** Pasteurization is a method of disinfection (not sterilization) used primarily for milk. The **Holder Method (Low-Temperature Holding)** involves heating milk to **63°C (145°F) for 30 minutes**, followed by rapid cooling. The primary goal is to eliminate common milk-borne pathogens like *Mycobacterium tuberculosis*, *Brucella*, and *Salmonella*. However, **Coxiella burnetii** (the causative agent of Q fever) is the most heat-resistant non-spore-forming pathogen found in milk. While the Holder method significantly reduces its numbers, it may not reliably kill 100% of *C. burnetii* organisms, making it the "limiting organism" for this process. **2. Why Other Options are Incorrect:** * **Option A & D:** These are incorrect because pasteurization is a **disinfection** process, not sterilization. It does not kill bacterial **spores** (e.g., *Bacillus* or *Clostridium* spores) or thermoduric bacteria. Therefore, it is false to say "all" bacteria or spores are destroyed. * **Option C:** This is a generic distractor. While pasteurization reduces the microbial load significantly (often by 97-99%), the specific clinical significance of the Holder method in microbiology exams is defined by its efficacy against specific pathogens like *C. burnetii*. **3. NEET-PG High-Yield Pearls:** * **Flash Method (HTST):** 72°C for 15 seconds. This is more common commercially. * **Ultra-High Temperature (UHT):** 135°C for 1-2 seconds (achieves sterility). * **Phosphatase Test:** Used to check the efficacy of pasteurization. If the enzyme phosphatase is destroyed, pasteurization is considered successful. * **Coxiella burnetii:** It is an obligate intracellular bacterium and is the most heat-resistant pathogen in milk.

Question 70: Which of the following agents are used to check the sufficiency of autoclaving, hydroclaving, and microwaving?

A. Autoclaving - Bacillus atrophaeus, Hydroclaving - Bacillus subtilis, Microwaving - Geobacillus stearothermophilus
B. Autoclaving - Bacillus subtilis, Hydroclaving - Bacillus atrophaeus, Microwaving - Geobacillus stearothermophilus
C. Autoclaving - Geobacillus stearothermophilus, Hydroclaving - Bacillus subtilis, Microwaving - Bacillus atrophaeus (Correct Answer)
D. Autoclaving - Bacillus atrophaeus, Hydroclaving - Geobacillus stearothermophilus, Microwaving - Bacillus subtilis

Explanation: ### Explanation The core concept behind biological indicators (BIs) is the use of highly resistant bacterial spores to validate that a sterilization process has successfully eliminated all microbial life. **1. Why the Correct Answer (C) is Right:** * **Autoclaving (Moist Heat):** Uses **_Geobacillus stearothermophilus_** (formerly *Bacillus stearothermophilus*). This thermophilic bacterium is highly resistant to moist heat, withstanding temperatures up to 121°C. If these spores are killed, it confirms the autoclave reached the necessary temperature and pressure parameters. * **Hydroclaving:** This is a steam-based fragmentation process used for biomedical waste. The standard biological indicator used to validate its efficacy is **_Bacillus subtilis_** (specifically the globigii strain). * **Microwaving:** In clinical waste management, microwaving relies on moist heat generated by microwave energy. The biological indicator used for validation is **_Bacillus atrophaeus_** (formerly *B. subtilis var. niger*). **2. Analysis of Incorrect Options:** * **Options A, B, and D** are incorrect because they mismatch the organism to the sterilization method. A common distractor is *Bacillus atrophaeus*, which is the gold standard for **Dry Heat Sterilization** (Hot Air Oven) and **Ethylene Oxide (ETO)**, but not for standard autoclaving. **3. High-Yield Clinical Pearls for NEET-PG:** * **Hot Air Oven / ETO:** *Bacillus atrophaeus* (NCTC 10073). * **Ionizing Radiation:** *Bacillus pumilus*. * **Plasma Sterilization (H₂O₂):** *Geobacillus stearothermophilus*. * **Filtration:** *Brevundimonas diminuta* (used to check the integrity of 0.22 μm filters). * **D-value:** The time required to kill 90% of a specific microorganism at a given temperature; it is the metric used to determine the resistance of these indicators.

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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