Identification: low copy number DNA: practice question

A Forensic Medicine 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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Forensic Medicine · Identification: low copy number DNA

Sample item

A very small number of skin cells recovered from a handled weapon are amplified with extra PCR cycles to obtain a profile. What is the main interpretative problem with such low copy number results?

AInability to distinguish between STR lociBAllele drop-out and contamination with extraneous DNACExcessive degradation of the Y chromosome onlyDFailure of any amplification at all

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

The answer and the reasoning

B. Allele drop-out and contamination with extraneous DNA

With few template molecules, stochastic effects cause alleles to drop out or extra alleles to appear from tiny amounts of contaminating DNA. The profile may therefore be partial or misleading. Strict contamination control and cautious interpretation are essential.

Option by option

A. Inability to distinguish between STR loci
Different STR loci remain distinguishable as they have different sizes and dyes.
B. Allele drop-out and contamination with extraneous DNA (correct)
Correct. Stochastic drop-out and drop-in from contamination limit the reliability of low copy results.
C. Excessive degradation of the Y chromosome only
Degradation does not selectively affect only the Y chromosome.
D. Failure of any amplification at all
Extra cycles are used precisely to allow amplification, which can occur.

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 Forensic Medicine question

What is the answer to this Forensic Medicine question?

The best answer is B, Allele drop-out and contamination with extraneous DNA. With few template molecules, stochastic effects cause alleles to drop out or extra alleles to appear from tiny amounts of contaminating DNA.

Why is "Inability to distinguish between STR loci" not the answer?

Different STR loci remain distinguishable as they have different sizes and dyes.

Why is "Excessive degradation of the Y chromosome only" not the answer?

Degradation does not selectively affect only the Y chromosome.

Why is "Failure of any amplification at all" not the answer?

Extra cycles are used precisely to allow amplification, which can occur.

What concept does this question test?

It tests Identification: low copy number DNA in Forensic Medicine. The profile may therefore be partial or misleading. Strict contamination control and cautious interpretation are essential.

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 Identification: low copy number DNA?

Learn postmortem changes with their timelines, and toxicology as poison, sign, test and antidote. Then use the flashcards and the other Forensic Medicine 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.