Disinfection: chlorhexidine: 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: chlorhexidine

Sample item

A surgeon is choosing a skin antiseptic for an elective operation. Which property of chlorhexidine makes it attractive for pre-operative skin preparation?

AIt acts by oxidising bacterial sulfhydryl groupsBIt is reliably sporicidal at the concentrations used on skinCIt binds the skin and gives persistent residual antibacterial activityDIt is the most effective agent against Mycobacterium tuberculosis

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

The answer and the reasoning

C. It binds the skin and gives persistent residual antibacterial activity

Chlorhexidine is a cationic biguanide that damages the cell membrane and binds to the stratum corneum, leaving residual activity for hours. It is active mainly against Gram-positive bacteria and less so against Gram-negative bacteria, and it is not sporicidal and poorly mycobactericidal. It is inactivated by anionic soaps.

Option by option

A. It acts by oxidising bacterial sulfhydryl groups
Oxidation of sulfhydryl groups is the mechanism of iodine and chlorine compounds, not of chlorhexidine.
B. It is reliably sporicidal at the concentrations used on skin
Chlorhexidine is not sporicidal at skin-antiseptic concentrations; glutaraldehyde or autoclaving is needed for spores.
C. It binds the skin and gives persistent residual antibacterial activity (correct)
Binding to the stratum corneum gives lasting residual activity, which is the key advantage of chlorhexidine.
D. It is the most effective agent against Mycobacterium tuberculosis
Chlorhexidine has weak activity against mycobacteria; glutaraldehyde or alcohol-based agents are better.

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 C, It binds the skin and gives persistent residual antibacterial activity. Chlorhexidine is a cationic biguanide that damages the cell membrane and binds to the stratum corneum, leaving residual activity for hours.

Why is "It acts by oxidising bacterial sulfhydryl groups" not the answer?

Oxidation of sulfhydryl groups is the mechanism of iodine and chlorine compounds, not of chlorhexidine.

Why is "It is reliably sporicidal at the concentrations used on skin" not the answer?

Chlorhexidine is not sporicidal at skin-antiseptic concentrations; glutaraldehyde or autoclaving is needed for spores.

Why is "It is the most effective agent against Mycobacterium tuberculosis" not the answer?

Chlorhexidine has weak activity against mycobacteria; glutaraldehyde or alcohol-based agents are better.

What concept does this question test?

It tests Disinfection: chlorhexidine in Microbiology. It is active mainly against Gram-positive bacteria and less so against Gram-negative bacteria, and it is not sporicidal and poorly mycobactericidal. It is inactivated by anionic soaps.

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: chlorhexidine?

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.