Molecular biology and genetics: cell-free fetal DNA screening: practice question

A Biochemistry 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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Biochemistry · Molecular biology and genetics: cell-free fetal DNA screening

Sample item

Non-invasive prenatal testing for trisomy 21 analyses cell-free DNA in the mother's plasma from about the tenth week. From which source does the fetal fraction of this DNA come?

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

The answer and the reasoning

C. Placental trophoblast cells undergoing apoptosis

Cell-free fetal DNA in maternal plasma comes mainly from apoptosis of placental trophoblasts and makes up about 10 percent of the cell-free DNA. Sequencing counts chromosome 21 fragments, and an excess suggests trisomy. It is a screening test, and a positive result needs confirmation by an invasive test.

Option by option

  • A. Maternal white cells lysed in the sample

    Maternal white cells supply the maternal fraction of the cell-free DNA, not the fetal fraction.

  • B. Fetal haematopoietic stem cells in the maternal circulation

    Fetal cells in the maternal circulation are very rare, and the test does not rely on them.

  • C. Placental trophoblast cells undergoing apoptosis (correct)

    Correct. The fetal fraction is released from placental cells and is detectable in maternal plasma.

  • D. Amniotic fluid cells

    Amniotic fluid cells are sampled by amniocentesis, which is an invasive diagnostic test.

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.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Placental trophoblast cells undergoing apoptosis. Cell-free fetal DNA in maternal plasma comes mainly from apoptosis of placental trophoblasts and makes up about 10 percent of the cell-free DNA.

Why is "Maternal white cells lysed in the sample" not the answer?

Maternal white cells supply the maternal fraction of the cell-free DNA, not the fetal fraction.

Why is "Fetal haematopoietic stem cells in the maternal circulation" not the answer?

Fetal cells in the maternal circulation are very rare, and the test does not rely on them.

Why is "Amniotic fluid cells" not the answer?

Amniotic fluid cells are sampled by amniocentesis, which is an invasive diagnostic test.

What concept does this question test?

It tests Molecular biology and genetics: cell-free fetal DNA screening in Biochemistry. Sequencing counts chromosome 21 fragments, and an excess suggests trisomy. It is a screening test, and a positive result needs confirmation by an invasive test.

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 Molecular biology and genetics: cell-free fetal DNA screening?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.