Sterilization and Disinfection — MCQs

Sterilization and Disinfection Indian Medical PG Practice Questions and MCQs

Question 171: Culture media containing egg or serum is typically sterilized by using which method?

A. Tyndallization (Correct Answer)
B. Autoclaving
C. Hot air oven
D. 5% Cresol

Explanation: **Explanation:** The correct answer is **Tyndallization** (Intermittent Sterilization). **1. Why Tyndallization is correct:** Culture media containing heat-sensitive organic constituents like **egg** (e.g., Lowenstein-Jensen medium) or **serum** (e.g., Loeffler’s serum slope) cannot withstand the high temperatures of autoclaving (121°C), which would cause protein coagulation and nutrient degradation. Tyndallization involves heating the medium at **100°C for 20–30 minutes on three successive days**. * **Day 1:** Kills vegetative forms. * **Interval:** Remaining spores germinate into vegetative forms. * **Day 2 & 3:** Kills the newly germinated vegetative bacteria. **2. Why other options are incorrect:** * **Autoclaving:** Uses pressurized steam at 121°C. While it is the "gold standard" for most media, it destroys the texture and nutritional value of egg and serum-based media. * **Hot Air Oven:** Uses dry heat (160°C). This is used for glassware and forceps; it would char or incinerate organic culture media. * **5% Cresol:** This is a chemical disinfectant used for surfaces and discarded biological waste, not for sterilizing growth media. **3. NEET-PG High-Yield Pearls:** * **Inspiration:** If the temperature is kept between **75°C–85°C** for 3 successive days (specifically for LJ medium), the process is called **Inspissation**. * **Membrane Filtration:** Preferred for antibiotic solutions, urea, and vitamins that are highly thermolabile. * **Flash Point:** Remember that serum is sterilized at 56°C for one hour for several days if Tyndallization is not used. * **LJ Medium:** Sterilized via Inspissator at 80°C for 2 hours for 3 days.

Question 172: Which biological indicator is used in an autoclave?

A. Clostridium tetani
B. Bacillus stereothermophilus (Correct Answer)
C. Bacillus pumilis
D. Bacillus subtilis Var Niger

Explanation: **Explanation:** **Biological indicators** are the most reliable method for monitoring the efficacy of sterilization because they test the process against the most resistant microbial forms: bacterial spores. **Why Bacillus stearothermophilus is correct:** Autoclaving (Moist Heat Sterilization) typically operates at **121°C for 15 minutes** at 15 psi. *Geobacillus stearothermophilus* (formerly *Bacillus stearothermophilus*) is the indicator of choice because it is a thermophilic aerobe that produces highly heat-resistant spores. If the autoclave cycle can kill these spores, it is assumed that all other vegetative pathogens and spores have been destroyed. **Analysis of Incorrect Options:** * **Clostridium tetani:** While it produces spores, it is a human pathogen and an anaerobe, making it unsafe and impractical for routine laboratory quality control. * **Bacillus pumilus:** This is the biological indicator used specifically for **Ionizing Radiation** (Gamma rays). * **Bacillus subtilis Var Niger (B. atrophaeus):** This is the biological indicator used for **Dry Heat Sterilization** (Hot Air Oven) and **Ethylene Oxide (ETO)** sterilization. **High-Yield Clinical Pearls for NEET-PG:** * **Sterilization Monitoring:** There are three types: Physical (pressure/temp gauges), Chemical (Browne’s tubes/Bowie-Dick tape), and Biological (most definitive). * **D-value:** This refers to the time required to kill 90% of organisms at a specific temperature. * **Flash Autoclaving:** Done at 134°C for 3 minutes for urgent surgical instruments. * **Culture Conditions:** After autoclaving, the *B. stearothermophilus* spores are incubated at **55–60°C** for up to 7 days (or rapid readout in 1–3 hours) to check for growth/acid production.

Question 173: Which of the following is NOT sporicidal?

A. Lysol (Correct Answer)
B. Glutaraldehyde
C. Ethylene oxide
D. Formaldehyde

Explanation: **Explanation:** The correct answer is **Lysol**. To answer this question, one must distinguish between **High-level disinfectants (Sterilants)** and **Intermediate/Low-level disinfectants**. 1. **Why Lysol is the correct answer:** Lysol is a brand name for **Cresol (a Phenolic compound)**. Phenols act by disrupting cell membranes and denaturing proteins. While they are effective against vegetative bacteria, fungi, and some viruses, they are **not sporicidal**. Therefore, they are classified as intermediate-level disinfectants and cannot be used for sterilization. 2. **Analysis of incorrect options (Sporicidal agents):** * **Glutaraldehyde (Cidex):** A high-level disinfectant that kills spores by alkylation of amino and hydroxyl groups. It is commonly used for "cold sterilization" of endoscopes (requires 10 hours for sporicidal action). * **Ethylene Oxide (ETO):** A potent gaseous sterilant used for heat-sensitive items (e.g., plastic syringes, heart-lung machines). It is highly sporicidal via alkylation. * **Formaldehyde:** A gas (or liquid as Formalin) that is sporicidal. It is used for fumigating operation theaters and preserving tissues. **NEET-PG High-Yield Pearls:** * **Sterilization vs. Disinfection:** Sterilization kills *all* microbial life, including spores; disinfection does not necessarily kill spores. * **Glutaraldehyde (2%):** The agent of choice for delicate instruments like cystoscopes and endoscopes. * **Phenol Coefficient:** A measure used to compare the disinfectant killing power of a substance to that of phenol. * **Chlorhexidine (Savlon):** Like Lysol, it is also **not sporicidal**.

Question 174: Hot air oven is used for sterilization of all except?

A. Glass wares
B. Dusting Powder
C. L.J. media (Correct Answer)
D. Liquid paraffin

Explanation: **Explanation:** The **Hot Air Oven (Dry Heat Sterilization)** operates at high temperatures (typically 160°C for 1 hour). This method is unsuitable for materials that are heat-sensitive or contain moisture, as high heat will denature proteins and destroy the integrity of the substance. **Why L.J. Media is the correct answer:** Lowenstein-Jensen (L.J.) media contains **egg yolk**, which is highly heat-sensitive. If subjected to the high temperatures of a hot air oven, the proteins would coagulate and the nutrients would be destroyed. Instead, L.J. media is sterilized via **Inspissation** (heating at 80-85°C for 30 minutes on three consecutive days), which solidifies the media without damaging its nutritional properties. **Analysis of incorrect options:** * **Glasswares (A):** Items like Petri dishes, flasks, and pipettes are the most common materials sterilized in a hot air oven because they can withstand high dry heat without breaking. * **Dusting Powder (B):** Dry heat is the method of choice for powders because steam (autoclave) cannot penetrate them effectively and would make them damp/clumpy. * **Liquid Paraffin (C):** Oils, fats, and greases are sterilized by dry heat because steam is unable to penetrate oily substances. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Dry heat kills microbes primarily through **protein oxidation** and oxidative damage. (Contrast: Autoclave kills via protein denaturation/coagulation). * **Sterilization Control:** The biological indicator used for a Hot Air Oven is ***Bacillus atrophaeus*** (formerly *B. subtilis var. niger*). * **Temperature/Time:** 160°C for 60 mins is the standard; 170°C for 30 mins or 180°C for 20 mins are alternatives. * **Sharp Instruments:** While glass is ideal, repeated dry heat can dull the cutting edges of surgical scalpels; however, it is still preferred over autoclaving for certain non-disposable sharps to prevent rusting.

Question 175: The disinfectant action of chlorine is MOSTLY due to which of the following forms?

A. Hypochlorous acid (Correct Answer)
B. Hypochlorite ion
C. Hydrochloric acid
D. Both Hypochlorous acid and Hypochlorite ion

Explanation: **Explanation:** The disinfectant action of chlorine in water depends on the formation of **Free Available Chlorine**. When chlorine (as gas or hypochlorite) is added to water, it reacts to form **Hypochlorous acid (HOCl)** and **Hypochlorite ions (OCl⁻)**. **1. Why Hypochlorous acid (HOCl) is the correct answer:** HOCl is the most active and potent germicidal form of chlorine. It is electrically neutral and possesses a small molecular size, allowing it to easily penetrate the negatively charged cell walls of microorganisms. Once inside, it destroys vital enzymes and proteins through oxidation. It is estimated to be **70–80 times more effective** at killing microbes than the hypochlorite ion. **2. Why other options are incorrect:** * **Hypochlorite ion (OCl⁻):** While it does have disinfectant properties, it is much less effective because its negative charge causes it to be repelled by the bacterial cell wall, hindering penetration. * **Hydrochloric acid (HCl):** This is a byproduct of the reaction but does not contribute significantly to the biocidal/disinfectant action; it primarily affects the pH of the solution. * **Both A and B:** While both exist in equilibrium, the question asks for the form "mostly" responsible for the action. HOCl is the primary active agent. **High-Yield Clinical Pearls for NEET-PG:** * **pH Dependency:** The efficacy of chlorine is highly pH-dependent. At a lower pH (pH 5–7), HOCl predominates, making disinfection more efficient. At a high pH (>8.5), OCl⁻ predominates, reducing efficacy. * **Contact Time:** For effective sterilization of water, a contact time of at least **30–60 minutes** is required. * **Free Residual Chlorine:** To ensure safety against subsequent contamination, the recommended free residual chlorine in drinking water is **0.5 mg/L**. * **Chlorine Demand:** This is the difference between the amount of chlorine added and the amount of free residual chlorine remaining after a specific contact period.

Question 176: Which of the following methods can be used for the disinfection of sputum?

A. Boiling, Sunlight, Burning
B. Boiling, Autoclaving, Burning (Correct Answer)
C. Boiling, Autoclaving, Sunlight
D. Boiling, Autoclaving

Explanation: **Explanation:** Sputum is a highly infectious biological material, often containing resilient pathogens like *Mycobacterium tuberculosis*. Effective disposal requires methods that ensure complete destruction of these microorganisms to prevent environmental contamination and nosocomial spread. **Why Option B is Correct:** * **Burning (Incineration):** This is the **gold standard** for disposing of infectious waste like sputum. It ensures complete destruction of the organic matter and the pathogens within. * **Autoclaving:** Using steam under pressure (121°C for 15 mins) is the most reliable method of sterilization, killing all vegetative forms and highly resistant spores. * **Boiling:** While technically a disinfection method rather than sterilization, boiling for 20–30 minutes is effective in killing most respiratory pathogens, including the tubercle bacilli, making it a viable option for sputum. **Analysis of Incorrect Options:** * **Options A & C (Sunlight):** While UV rays in sunlight have some germicidal properties, sunlight is **unreliable and slow**. It cannot penetrate thick, mucoid sputum effectively and is never recommended as a primary disinfection method for clinical waste. * **Option D (Incomplete):** While Boiling and Autoclaving are correct, this option is less comprehensive than Option B, which includes Burning—the most preferred clinical method. **NEET-PG High-Yield Pearls:** * **Sputum Disposal:** In a clinical setting, sputum is often collected in disposable cups and **incinerated**. If cups are reusable, they must be **autoclaved**. * **Chemical Disinfection:** If heat is unavailable, **5% Cresol** or **1% Sodium Hypochlorite** can be used for sputum disinfection (contact time: 1 hour). * **Pre-treatment:** Never discard sputum directly into drains without prior disinfection or autoclaving.

Question 177: Hepatitis A virus can be inactivated by which of the following methods?

A. Boiling at 120°C for 1 minute (Correct Answer)
B. Ether
C. Chloroform
D. Iodine

Explanation: **Explanation:** Hepatitis A Virus (HAV) is a non-enveloped, single-stranded RNA virus belonging to the *Picornaviridae* family. Its lack of a lipid envelope makes it remarkably stable and resistant to many common physical and chemical agents. **1. Why Option A is Correct:** HAV is highly heat-resistant compared to other viruses. While it can survive at 60°C for an hour, it is effectively inactivated by high temperatures. **Boiling at 100°C–120°C for at least 1 minute** (or autoclaving) is a reliable method to denature the viral capsid and render the virus non-infectious. This is a critical point for preventing fecal-oral transmission via contaminated water or food. **2. Why Incorrect Options are Wrong:** * **Ether and Chloroform (Options B & C):** These are organic solvents that act by dissolving the lipid envelope of viruses. Because HAV is a **non-enveloped virus**, it is inherently resistant to lipid solvents like ether, chloroform, and alcohols. * **Iodine (Option D):** While iodine is a potent antiseptic, HAV shows significant resistance to standard concentrations of many disinfectants. Effective chemical inactivation usually requires specific concentrations of chlorine (sodium hypochlorite), formalin, or glutaraldehyde. **High-Yield Clinical Pearls for NEET-PG:** * **Resistance Profile:** HAV is resistant to low pH (pH 1), allowing it to survive the gastric acid barrier. * **Chlorination:** In water treatment, HAV is inactivated by free residual chlorine (0.5–1.5 mg/L) after 30 minutes of contact. * **Comparison:** Unlike Hepatitis B (HBV) or C (HCV), which are enveloped and susceptible to organic solvents, HAV and HEV (both fecal-oral) are non-enveloped and environmentally hardy. * **Inactivation Summary:** Formalin (0.3% at 37°C for 72 hours) and UV radiation are also effective against HAV.

Question 178: Who introduced the technique of sterilization?

A. Robert Koch
B. Edward Jenner
C. Louis Pasteur (Correct Answer)
D. Lister

Explanation: ### Explanation **Correct Answer: C. Louis Pasteur** **Louis Pasteur** is credited with introducing the concept and techniques of sterilization. His experiments disproved the theory of "spontaneous generation" and established the **Germ Theory of Disease**. He demonstrated that microorganisms could be killed by heat, leading to the development of **Pasteurization** (used for milk) and the early principles of steam sterilization (autoclaving) and hot air sterilization. His work laid the foundation for modern microbiology and aseptic techniques. **Analysis of Incorrect Options:** * **A. Robert Koch:** Known as the "Father of Bacteriology," he is famous for **Koch’s Postulates**, identifying the causative agents of Anthrax, Tuberculosis, and Cholera, and introducing the use of solid culture media (Agar). * **B. Edward Jenner:** Known as the "Father of Immunology," he developed the first vaccine (for **Smallpox**) using the cowpox virus. * **D. Joseph Lister:** Known as the "Father of Antiseptic Surgery," he introduced the use of **Carbolic acid (Phenol)** to disinfect wounds and surgical instruments, significantly reducing post-operative infections. **High-Yield Clinical Pearls for NEET-PG:** * **Father of Microbiology:** Louis Pasteur. * **Father of Modern Antisepsis:** Joseph Lister (introduced the Phenol Coefficient). * **Sterilization vs. Disinfection:** Sterilization kills *all* forms of microbial life, including highly resistant **bacterial spores**, whereas disinfection does not necessarily eliminate spores. * **Autoclaving (Steam under pressure):** The most reliable method of sterilization; standard conditions are **121°C for 15 minutes at 15 psi**. * **Hot Air Oven:** Uses dry heat; standard conditions are **160°C for 2 hours**.

Question 179: What concentration of ethyl alcohol is effective against spores?

A. 70% (Correct Answer)
B. 60%
C. 50%
D. 100%

Explanation: **Explanation:** The correct answer is **70%**. **Mechanism of Action:** Ethyl alcohol (ethanol) acts by denaturing proteins and dissolving lipid membranes. The presence of water is crucial for this process because water acts as a catalyst in the denaturation of proteins. At a concentration of **70%**, the water content allows the alcohol to penetrate the cell wall and reach the internal proteins of the microorganism effectively. **Why 70% is correct:** While the question asks about effectiveness against "spores," it is a high-yield clinical fact that **alcohols are generally NOT sporicidal.** However, in the context of standard medical examinations like NEET-PG, 70% is the "optimal" germicidal concentration. It is effective against vegetative bacteria, fungi, and enveloped viruses. If a question implies any level of efficacy or asks for the standard disinfectant concentration, 70% is the gold standard. **Analysis of Incorrect Options:** * **60% and 50%:** These concentrations are less effective because the alcohol content is too low to cause rapid protein denaturation and membrane disruption. * **100% (Absolute Alcohol):** Paradoxically, 100% alcohol is less effective than 70%. Without water, absolute alcohol causes rapid coagulation of surface proteins, creating a protective "shell" around the microorganism that prevents the alcohol from penetrating deeper into the cell. **High-Yield Clinical Pearls for NEET-PG:** * **Sporicidal Activity:** Alcohols (Ethyl and Isopropyl) are **not** sporicidal. To kill spores, agents like Glutaraldehyde (2%), Formaldehyde, or Autoclaving are required. * **Isopropyl Alcohol:** Often preferred over ethanol for skin antisepsis as it is a better fat solvent and more bactericidal. * **Contact Time:** For effective disinfection, 70% alcohol requires a contact time of at least 10–30 seconds. * **Virucidal Spectrum:** Alcohol is effective against enveloped viruses (HIV, HBV) but less effective against non-enveloped viruses (Enteroviruses).

Question 180: Cystoscopes and bronchoscopes are sterilized by which of the following methods?

A. Orthophthaldehyde (Correct Answer)
B. Glutaraldehyde
C. Formaldehyde
D. Isopropyl alcohol

Explanation: **Explanation:** The correct answer is **Orthophthaldehyde (OPA)**. In modern clinical practice, endoscopes (such as cystoscopes and bronchoscopes) are classified as **semi-critical items** according to the Spaulding classification. These items come into contact with mucous membranes and require **High-Level Disinfection (HLD)**. 1. **Why Orthophthaldehyde (OPA) is correct:** OPA (0.55%) has largely replaced glutaraldehyde as the gold standard for HLD of heat-sensitive endoscopes. It is superior because it is more stable, does not require activation (no mixing), has a faster kill time (5–12 minutes), and lacks the pungent odor and irritating vapors associated with glutaraldehyde. 2. **Why Glutaraldehyde is incorrect:** While traditionally used (Cidex 2%), it is now a second-line choice due to its respiratory toxicity, potential to cause skin sensitization, and the requirement for a 20-minute immersion time. It also requires "activation" by adding a buffer. 3. **Why Formaldehyde is incorrect:** It is primarily used for preserving tissues or fumigating rooms. It is highly toxic, carcinogenic, and too slow-acting for routine instrument disinfection. 4. **Why Isopropyl alcohol is incorrect:** Alcohols are intermediate-level disinfectants. They are ineffective against spores and many hydrophilic viruses, making them unsuitable for semi-critical instruments like bronchoscopes. **High-Yield Clinical Pearls for NEET-PG:** * **Spaulding Classification:** Critical (Sterilization), Semi-critical (HLD), Non-critical (Low-level disinfection). * **OPA Advantage:** It is effective against glutaraldehyde-resistant mycobacteria. * **Sterilization of choice for Robotic instruments:** Hydrogen peroxide plasma (Sterrad). * **Prion disinfection:** Autoclaving at 134°C for 18 minutes or 1N NaOH for 1 hour.

Practice by Chapter

Physical Methods of Sterilization

Practice Questions

Chemical Methods of Sterilization

Practice Questions

Disinfectants and Antiseptics

Practice Questions

Sterilization Monitoring

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Hospital Sterilization Protocols

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Surgical Instruments Sterilization

Practice Questions

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

Practice Questions

Regulatory Aspects of Sterilization

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