Medical genetics: triploidy: practice question

A Anatomy 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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Anatomy · Medical genetics: triploidy

Sample item

A woman at 11 weeks has vaginal bleeding. Ultrasound shows a thickened placenta with cystic spaces and an abnormal fetus. Tissue karyotyping shows 69,XXY. How did this arise?

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

The answer and the reasoning

C. Fertilisation of one ovum by two sperm

A partial hydatidiform mole is usually triploid, with two paternal haploid sets and one maternal set, most often from dispermy. A complete mole is diploid and entirely paternal, and it has no fetus.

Option by option

  • A. Duplication of one sperm genome in an empty ovum

    Duplication of one sperm genome in an empty ovum gives a diploid, all-paternal complete mole.

  • B. Meiotic nondisjunction in the oocyte

    Oocyte nondisjunction gives a trisomy or monosomy, not a full triploid set.

  • C. Fertilisation of one ovum by two sperm (correct)

    Correct. Dispermy gives two paternal and one maternal haploid set, producing a triploid partial mole.

  • D. Fertilisation of two ova by one sperm

    One sperm cannot fertilise two ova, and two ova would give twins.

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 Anatomy question

What is the answer to this Anatomy question?

The best answer is C, Fertilisation of one ovum by two sperm. A partial hydatidiform mole is usually triploid, with two paternal haploid sets and one maternal set, most often from dispermy.

Why is "Duplication of one sperm genome in an empty ovum" not the answer?

Duplication of one sperm genome in an empty ovum gives a diploid, all-paternal complete mole.

Why is "Meiotic nondisjunction in the oocyte" not the answer?

Oocyte nondisjunction gives a trisomy or monosomy, not a full triploid set.

Why is "Fertilisation of two ova by one sperm" not the answer?

One sperm cannot fertilise two ova, and two ova would give twins.

What concept does this question test?

It tests Medical genetics: triploidy in Anatomy. A complete mole is diploid and entirely paternal, and it has no fetus.

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 Medical genetics: triploidy?

Learn every nerve by its route, its muscles and what a lesion looks like at the bedside. That one habit answers a large share of questions. Then use the flashcards and the other Anatomy 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.