Silver preparations are still used clinically chiefly for
Answer: C
Silver's activity at very low concentration, its oligodynamic action, suits it to prolonged contact with a large raw surface where infection risk is high. It is applied as a cream or impregnated into a dressing rather than used as a general disinfectant.
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Crystal violet and related dyes act mainly against
Answer: D
These basic dyes bind more readily to Gram-positive organisms, which is the same selectivity that makes them useful in staining. Their antimicrobial use is limited and largely topical, having been displaced by more reliable agents.
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Formaldehyde gas has been used to
Answer: A
As a gas it reaches surfaces throughout an enclosed space, which suited it to decontaminating rooms and cabinets, though its irritancy and carcinogenicity have narrowed its use. Vaporised hydrogen peroxide now serves much the same purpose more safely.
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An agent described as sporicidal is one that
Answer: B
Killing the dormant spore is the hardest test a chemical agent faces, and only a few classes manage it: the aldehydes, ethylene oxide, strong oxidising agents and high-strength halogens. Preventing sporulation is a quite different matter.
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Which one of the following is inactivated by hard water?
Answer: C
Correct answer: Quaternary ammonium compounds
Quaternary ammonium compounds are cationic surfactants, so the calcium and magnesium ions in hard water combine with them and reduce the active concentration. They are also antagonised by soaps, which are anionic, and by organic matter - three limitations that between them explain why they are used on clean, low-risk surfaces rather than as high-level disinfectants.
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The main reason ethylene oxide sterilization takes many hours is the need for
Answer: D
Correct answer: a long aeration period to remove toxic residue
The exposure itself takes a few hours, but ethylene oxide is absorbed by plastics and rubber and is toxic, mutagenic and irritant, so the load must then be aerated for many hours - sometimes over a day - before it is safe to handle or implant. That aeration dominates the total cycle time and is the method’s chief practical drawback.
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Vaporised hydrogen peroxide is attractive for room decontamination because it
Answer: A
Decomposing into harmless products means there is nothing toxic to remove afterwards, unlike formaldehyde or ethylene oxide. It is sporicidal at working concentrations, so it can decontaminate a room rather than merely clean it.
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Membrane filtration does not guarantee a sterile product because
Answer: B
A 0.22 micrometre pore retains vegetative bacteria but is wide open to anything smaller, so a filtered solution is bacteria-free rather than sterile. Removing viruses needs a much finer membrane or a different method altogether.
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Autoclave tape that has changed colour tells the user that
Answer: C
Correct answer: the pack has been exposed to heat
Autoclave tape is a process indicator: its stripes darken on reaching a certain temperature, so it distinguishes a processed pack from an unprocessed one. It says nothing about how long that temperature was held, and so nothing about sterility. Confirming that requires a chemical integrator inside the pack and, periodically, a biological indicator using spores of a resistant organism.
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Pasteurized milk still requires refrigeration because
Answer: D
Pasteurization is deliberately mild, so heat-tolerant spoilage organisms and any spores present remain alive. Cold slows their growth, which is why the product has a limited shelf life while sterilized milk keeps unopened at room temperature.
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