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Physical and Chemical Agents Questions and Answers

81.

The temperature of saturated steam rises above 100 degrees Celsius only when

Answer: A

Correct answer: the pressure is raised above atmospheric

Saturated steam sits at the boiling point corresponding to its pressure, and at atmospheric pressure that is 100 degrees however much heat is supplied. Raising the pressure raises the boiling point, so at 15 pounds per square inch above atmospheric the steam is at 121 degrees. The pressure is therefore only the means; it is the temperature that sterilises.

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82.

Superheated steam is less effective than saturated steam because it

Answer: B

Killing depends on steam condensing on the load, releasing its latent heat and wetting the surface. Steam heated beyond saturation stays gaseous, so it acts as dry heat and would need far higher temperatures and times.

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83.

Radiation sterilization of disposable plastic syringes is carried out with

Answer: C

Gamma radiation passes through the finished sealed package without heating it, so the item is sterilized in its final wrapping and stays sterile until opened. Ultraviolet light is stopped by the packaging and would never reach the contents.

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84.

Which one of the following pairs is correctly matched?

Answer: D

Ethylene oxide adds alkyl groups to nucleic acid and protein, which is why it is sporicidal. Alcohol denatures protein and dissolves lipid, hypochlorite oxidises, and quaternary compounds disorganise the membrane as cationic surfactants.

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85.

Filtration sterilizes a solution by

Answer: A

Correct answer: removing the organisms rather than killing them

A membrane filter of 0.22 micrometre pore size retains bacteria and fungi physically, so the filtrate is free of them while nothing has been killed. That is why filtration is used for heat-labile solutions - sera, antibiotic solutions, vitamins - and why it is not reliable against viruses or mycoplasmas, which are small enough to pass.

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86.

The commonest cause of autoclave failure in practice is

Answer: B

Trapped air keeps steam from the surfaces it must condense on, so parts of the load never reach the sterilizing conditions even though the gauge reads correctly. Prevacuum sterilizers and correct packing both exist to prevent it.

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