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Physical and Chemical Agents Questions and Answers

51.

The oligodynamic action of heavy metals refers to their

Answer: A

Correct answer: antimicrobial effect at very low concentrations

Oligodynamic action is the inhibition of microorganisms by metal ions at concentrations far below those needed for any chemical bulk effect - silver being the classic example, and the reason silver-impregnated dressings and catheters are used. The ions bind to sulphydryl groups in microbial enzymes, denaturing them at trace concentrations.

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52.

Quaternary ammonium compounds are

Answer: B

Being positively charged surfactants they adsorb to the negatively charged cell surface and disorganise the membrane. They are low-level disinfectants: pleasant to use and non-corrosive, but inactive against spores and unreliable against mycobacteria.

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53.

Quaternary ammonium compounds are antagonised by

Answer: C

Anionic soap carries the opposite charge and neutralises a cationic surfactant, so residual soap on a surface destroys the disinfectant's activity. Rinsing between cleaning and disinfection is therefore essential when these agents are used.

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54.

Glutaraldehyde and formaldehyde kill microorganisms by

Answer: D

Aldehydes add alkyl groups to amino and other reactive groups, cross-linking proteins and nucleic acids. Because that action extends to spores, a two per cent glutaraldehyde solution counts as a high-level disinfectant and, with long enough contact, a chemical sterilant.

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55.

Glutaraldehyde is usually employed at a concentration of about

Answer: A

Two per cent alkaline glutaraldehyde is the standard preparation for heat-sensitive instruments such as endoscopes. It is sporicidal on prolonged contact, and its irritant vapour is the reason it must be used in a ventilated area and rinsed off thoroughly.

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56.

Ethylene oxide is valuable as a sterilant because it

Answer: B

As a gas it penetrates packaging and complex devices and works near ambient temperature, so plastics and electronics survive. The drawbacks are a cycle measured in hours, flammability, and toxic residues that demand a long aeration period afterwards.

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57.

Ethylene oxide kills microorganisms by

Answer: C

Like the aldehydes it is an alkylating agent, adding groups to nucleic acid and protein and destroying their function, which makes it sporicidal. That same reactivity is why it is a recognised carcinogen and needs careful handling.

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58.

Hydrogen peroxide and peracetic acid act as

Answer: D

Both generate reactive oxygen species that attack lipids, proteins and nucleic acid, and both break down to harmless products, which is why vaporised hydrogen peroxide is attractive for room and device decontamination. At sufficient strength they are sporicidal.

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59.

Soaps and detergents contribute to infection control largely by

Answer: A

Their surfactant action loosens grease and dirt so that organisms are carried away in the rinse water. The value of handwashing lies chiefly in that removal, which is why it works against spores that no hand rub will kill.

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60.

An autoclave load must not be packed tightly because

Answer: B

Correct answer: steam has to reach every surface to be effective

Autoclaving works by saturated steam condensing on a surface and giving up its latent heat there, so any surface the steam cannot reach is simply not sterilised. A tightly packed chamber traps residual air, which both blocks contact and lowers the temperature the steam attains, so the load must be arranged to let steam circulate and air escape.

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