Medical genetics: isochromosome: 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: isochromosome

Sample item

A girl with short stature and features of Turner syndrome has the karyotype 46,X,i(Xq). What is the structural change?

ALoss of the short arm and duplication of the long arm of the X chromosomeBLoss of the long arm and duplication of the short arm of the X chromosomeCA ring formed after loss of both chromosome endsDTranslocation of the X chromosome to an autosome

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

The answer and the reasoning

A. Loss of the short arm and duplication of the long arm of the X chromosome

An isochromosome forms when the centromere divides transversely rather than longitudinally, giving a chromosome with two identical arms. In i(Xq) the short arm is lost and the long arm is duplicated. Loss of Xp, which contains the SHOX gene, explains the short stature.

Option by option

A. Loss of the short arm and duplication of the long arm of the X chromosome (correct)
Correct. i(Xq) has two long arms and no short arm.
B. Loss of the long arm and duplication of the short arm of the X chromosome
A duplicated short arm with lost long arm would be i(Xp), not i(Xq).
C. A ring formed after loss of both chromosome ends
A ring chromosome is a different structural change, written r(X).
D. Translocation of the X chromosome to an autosome
A translocation would be written t(X;A) and does not produce two identical arms.

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 Anatomy question

What is the answer to this Anatomy question?

The best answer is A, Loss of the short arm and duplication of the long arm of the X chromosome. An isochromosome forms when the centromere divides transversely rather than longitudinally, giving a chromosome with two identical arms.

Why is "Loss of the long arm and duplication of the short arm of the X chromo…" not the answer?

A duplicated short arm with lost long arm would be i(Xp), not i(Xq).

Why is "A ring formed after loss of both chromosome ends" not the answer?

A ring chromosome is a different structural change, written r(X).

Why is "Translocation of the X chromosome to an autosome" not the answer?

A translocation would be written t(X;A) and does not produce two identical arms.

What concept does this question test?

It tests Medical genetics: isochromosome in Anatomy. In i(Xq) the short arm is lost and the long arm is duplicated. Loss of Xp, which contains the SHOX gene, explains the short stature.

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: isochromosome?

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.