Embryology: limb patterning: 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 · Embryology: limb patterning

Sample item

In a chick embryo, grafting mesenchyme from the posterior margin of one limb bud onto the anterior margin of another produces mirror-image digit duplication. Which signalling molecule does this mesenchyme secrete?

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

The answer and the reasoning

C. Sonic hedgehog

The zone of polarising activity in the posterior limb bud secretes Sonic hedgehog, which sets the anteroposterior axis. FGF8 from the apical ectodermal ridge drives proximodistal outgrowth, and Wnt7a from the dorsal ectoderm patterns the dorsoventral axis.

Option by option

  • A. Fibroblast growth factor 8

    FGF8 from the apical ectodermal ridge maintains proximodistal outgrowth.

  • B. Bone morphogenetic protein 4

    BMP4 is involved in interdigital cell death rather than axis polarity.

  • C. Sonic hedgehog (correct)

    Correct. Sonic hedgehog from the zone of polarising activity specifies the anteroposterior axis.

  • D. Wnt7a

    Wnt7a from the dorsal ectoderm specifies the dorsal limb.

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.

More Anatomy questions

Questions about this Anatomy question

What is the answer to this Anatomy question?

The best answer is C, Sonic hedgehog. The zone of polarising activity in the posterior limb bud secretes Sonic hedgehog, which sets the anteroposterior axis.

Why is "Fibroblast growth factor 8" not the answer?

FGF8 from the apical ectodermal ridge maintains proximodistal outgrowth.

Why is "Bone morphogenetic protein 4" not the answer?

BMP4 is involved in interdigital cell death rather than axis polarity.

Why is "Wnt7a" not the answer?

Wnt7a from the dorsal ectoderm specifies the dorsal limb.

What concept does this question test?

It tests Embryology: limb patterning in Anatomy. FGF8 from the apical ectodermal ridge drives proximodistal outgrowth, and Wnt7a from the dorsal ectoderm patterns the dorsoventral axis.

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 Embryology: limb patterning?

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.