Clinical microbiology: real-time PCR: 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: real-time PCR

Sample item

Two plasma samples are tested by real-time PCR for a viral load using the same assay with 100% amplification efficiency. Sample A has a cycle threshold of 22 and sample B a cycle threshold of 32. How do their starting amounts of target compare?

ASample B has about 1,000 times more target than sample ABSample A has about 1,000 times more target than sample BCSample A has about 10 times more target than sample BDThey are equal because both are positive

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

The answer and the reasoning

B. Sample A has about 1,000 times more target than sample B

The cycle threshold is the cycle at which fluorescence crosses the threshold, and it is inversely related to the starting amount of target. With perfect efficiency the target doubles every cycle, so a difference of 10 cycles equals 2 to the power 10, or about 1,000-fold. A lower threshold means more starting target.

Option by option

A. Sample B has about 1,000 times more target than sample A
This reverses the relationship, since the later threshold of sample B means less target.
B. Sample A has about 1,000 times more target than sample B (correct)
Correct. A lower cycle threshold by 10 cycles means about 1,000 times more target.
C. Sample A has about 10 times more target than sample B
Ten times would apply to a difference of about 3.3 cycles, not 10.
D. They are equal because both are positive
Positivity is qualitative, whereas the cycle threshold carries quantitative information.

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 B, Sample A has about 1,000 times more target than sample B. The cycle threshold is the cycle at which fluorescence crosses the threshold, and it is inversely related to the starting amount of target.

Why is "Sample B has about 1,000 times more target than sample A" not the answer?

This reverses the relationship, since the later threshold of sample B means less target.

Why is "Sample A has about 10 times more target than sample B" not the answer?

Ten times would apply to a difference of about 3.3 cycles, not 10.

Why is "They are equal because both are positive" not the answer?

Positivity is qualitative, whereas the cycle threshold carries quantitative information.

What concept does this question test?

It tests Clinical microbiology: real-time PCR in Microbiology. With perfect efficiency the target doubles every cycle, so a difference of 10 cycles equals 2 to the power 10, or about 1,000-fold. A lower threshold means more starting target.

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: real-time PCR?

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.