Clinical microbiology and hospital infection control: PCR contamination control: 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 · Clinical microbiology and hospital infection control: PCR contamination control

Sample item

A molecular laboratory repeatedly obtains false-positive PCR results because of amplicon carry-over. Which measure is a recognised way to prevent this?

ARaise the extension time of each cycleBRun more amplification cycles to dilute contaminationCUse a larger volume of sampleDSeparate pre- and post-amplification areas and use uracil-DNA glycosylase

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

The answer and the reasoning

D. Separate pre- and post-amplification areas and use uracil-DNA glycosylase

Amplified products are the main source of false positives. Physical separation of pre- and post-PCR work areas, one-way workflow, and incorporating dUTP with uracil-DNA glycosylase to destroy previous amplicons prevent carry-over.

Option by option

A. Raise the extension time of each cycle
Longer extension does not address contaminating product.
B. Run more amplification cycles to dilute contamination
More cycles amplify contaminating template further.
C. Use a larger volume of sample
More sample does not remove amplicon contamination.
D. Separate pre- and post-amplification areas and use uracil-DNA glycosylase (correct)
Correct. Workflow separation and UDG digestion of previous amplicons prevent carry-over.

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 D, Separate pre- and post-amplification areas and use uracil-DNA glycosylase. Amplified products are the main source of false positives.

Why is "Raise the extension time of each cycle" not the answer?

Longer extension does not address contaminating product.

Why is "Run more amplification cycles to dilute contamination" not the answer?

More cycles amplify contaminating template further.

Why is "Use a larger volume of sample" not the answer?

More sample does not remove amplicon contamination.

What concept does this question test?

It tests Clinical microbiology and hospital infection control: PCR contamination control in Microbiology. Physical separation of pre- and post-PCR work areas, one-way workflow, and incorporating dUTP with uracil-DNA glycosylase to destroy previous amplicons prevent carry-over.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Clinical microbiology and hospital infection control: PCR contamination control?

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.