Medical genetics: spinal muscular atrophy: 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: spinal muscular atrophy

Sample item

A 6-month-old infant is floppy with tongue fasciculations, areflexia and a bell-shaped chest, but is alert with normal sensation. Which neurons are lost and which gene is mutated?

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

The answer and the reasoning

B. Anterior horn cells; SMN1 gene

Spinal muscular atrophy is an autosomal recessive loss of lower motor neurons in the anterior horn and brainstem motor nuclei, due to deletion of SMN1 on chromosome 5. The infant is hypotonic and areflexic with normal cognition and sensation.

Option by option

  • A. Purkinje cells; ATM gene

    Purkinje cell loss with ATM mutation causes ataxia-telangiectasia, which presents with ataxia and not floppy areflexia.

  • B. Anterior horn cells; SMN1 gene (correct)

    Correct. Loss of anterior horn cells with SMN1 mutation explains the floppy, areflexic, alert infant.

  • C. Dorsal root ganglion cells; PMP22 gene

    Dorsal root ganglion loss would cause sensory deficits, which are absent here.

  • D. Corticospinal neurons; SPG gene

    Corticospinal neuron loss would give spasticity and hyperreflexia, not areflexia.

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 B, Anterior horn cells; SMN1 gene. Spinal muscular atrophy is an autosomal recessive loss of lower motor neurons in the anterior horn and brainstem motor nuclei, due to deletion of SMN1 on chromosome 5.

Why is "Purkinje cells; ATM gene" not the answer?

Purkinje cell loss with ATM mutation causes ataxia-telangiectasia, which presents with ataxia and not floppy areflexia.

Why is "Dorsal root ganglion cells; PMP22 gene" not the answer?

Dorsal root ganglion loss would cause sensory deficits, which are absent here.

Why is "Corticospinal neurons; SPG gene" not the answer?

Corticospinal neuron loss would give spasticity and hyperreflexia, not areflexia.

What concept does this question test?

It tests Medical genetics: spinal muscular atrophy in Anatomy. The infant is hypotonic and areflexic with normal cognition and sensation.

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: spinal muscular atrophy?

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.