General microbiology and sterilisation: cationic disinfectants and soap: 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 · General microbiology and sterilisation: cationic disinfectants and soap

Sample item

A ward uses benzalkonium chloride to disinfect a trolley that was cleaned earlier with ordinary soap and not rinsed. The disinfection is poor. What is the most likely explanation?

AAnionic soap neutralises the cationic surface-active disinfectantBSoap increases the penetration of the disinfectant into sporesCThe disinfectant is inactivated by sunlight on the trolleyDSoap residue is a nutrient that makes spores germinate

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

The answer and the reasoning

A. Anionic soap neutralises the cationic surface-active disinfectant

Quaternary ammonium compounds such as cetrimide and benzalkonium chloride are cationic detergents and are inactivated by anionic soaps, organic matter and hard water. Surfaces should be cleaned and rinsed before application. They are also poorly active against spores and non-enveloped viruses.

Option by option

A. Anionic soap neutralises the cationic surface-active disinfectant (correct)
Correct. The negatively charged soap binds and neutralises the positively charged disinfectant.
B. Soap increases the penetration of the disinfectant into spores
Soap does not help any disinfectant to penetrate spores, and quaternary compounds are not sporicidal.
C. The disinfectant is inactivated by sunlight on the trolley
Light is not the reason for loss of activity here.
D. Soap residue is a nutrient that makes spores germinate
Residue does not nutritionally defeat the disinfectant and spores are not germinated by soap.

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.

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is A, Anionic soap neutralises the cationic surface-active disinfectant. Quaternary ammonium compounds such as cetrimide and benzalkonium chloride are cationic detergents and are inactivated by anionic soaps, organic matter and hard water.

Why is "Soap increases the penetration of the disinfectant into spores" not the answer?

Soap does not help any disinfectant to penetrate spores, and quaternary compounds are not sporicidal.

Why is "The disinfectant is inactivated by sunlight on the trolley" not the answer?

Light is not the reason for loss of activity here.

Why is "Soap residue is a nutrient that makes spores germinate" not the answer?

Residue does not nutritionally defeat the disinfectant and spores are not germinated by soap.

What concept does this question test?

It tests General microbiology and sterilisation: cationic disinfectants and soap in Microbiology. Surfaces should be cleaned and rinsed before application. They are also poorly active against spores and non-enveloped viruses.

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 General microbiology and sterilisation: cationic disinfectants and soap?

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.