Disinfection: alcohols: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Microbiology · Disinfection: alcohols

Sample item

A student asks why 70% ethanol is used for skin preparation instead of absolute (100%) ethanol. What is the correct explanation?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Water aids protein denaturation and penetration, whereas absolute alcohol coagulates surface proteins and forms a barrier

Protein denaturation by alcohol requires water. Absolute alcohol rapidly coagulates the outer layer of microbial protein, which blocks further penetration, so it kills less effectively. Alcohols at 60 to 90% are bactericidal, virucidal for enveloped viruses and fungicidal, but are not sporicidal.

Option by option

  • A. Absolute alcohol is sporicidal and is therefore unsafe for skin

    Alcohols are not sporicidal at any concentration, and skin safety is not the reason.

  • B. Water aids protein denaturation and penetration, whereas absolute alcohol coagulates surface proteins and forms a barrier (correct)

    Correct. A water content of about 30% gives optimal denaturation and penetration.

  • C. Seventy percent ethanol kills by alkylating nucleic acids

    Alkylation is the mechanism of ethylene oxide and formaldehyde, not of alcohols.

  • D. Absolute alcohol is ionised and produces toxic metabolites on skin

    Ethanol is not ionised on the skin and absorption through intact skin is negligible; this is not the basis.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

More Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is B, Water aids protein denaturation and penetration, whereas absolute alcohol coagulates surface proteins and forms a barrier. Protein denaturation by alcohol requires water.

Why is "Absolute alcohol is sporicidal and is therefore unsafe for skin" not the answer?

Alcohols are not sporicidal at any concentration, and skin safety is not the reason.

Why is "Seventy percent ethanol kills by alkylating nucleic acids" not the answer?

Alkylation is the mechanism of ethylene oxide and formaldehyde, not of alcohols.

Why is "Absolute alcohol is ionised and produces toxic metabolites on skin" not the answer?

Ethanol is not ionised on the skin and absorption through intact skin is negligible; this is not the basis.

What concept does this question test?

It tests Disinfection: alcohols in Microbiology. Absolute alcohol rapidly coagulates the outer layer of microbial protein, which blocks further penetration, so it kills less effectively. Alcohols at 60 to 90% are bactericidal, virucidal for enveloped viruses and fungicidal, but are not sporicidal.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Disinfection: alcohols?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.