Neuroanatomy: spinothalamic lamination: 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 · Neuroanatomy: spinothalamic lamination

Sample item

A patient with an extramedullary tumour compressing the cord from outside first loses pain and temperature in the sacral dermatomes, and the level later rises. What explains this pattern?

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

The answer and the reasoning

B. Sacral fibres lie most laterally in the lateral spinothalamic tract

In the lateral spinothalamic tract the fibres are laminated, with sacral fibres lateral and cervical fibres medial. An external compression hits the lateral fibres first, so loss ascends from the sacral level. An intramedullary lesion such as syringomyelia affects medial fibres first and may spare the sacral region.

Option by option

  • A. Sacral fibres run in the dorsal columns

    The dorsal columns carry touch and vibration, not pain and temperature.

  • B. Sacral fibres lie most laterally in the lateral spinothalamic tract (correct)

    Correct. Lateral sacral fibres are compressed first by an extrinsic mass.

  • C. Sacral fibres lie most medially in the lateral spinothalamic tract

    Sacral fibres are not medial; the medial fibres are cervical.

  • D. Sacral fibres decussate lower than other fibres and are compressed first

    All spinothalamic fibres cross within a segment or two of entry, so crossing level does not explain it.

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, Sacral fibres lie most laterally in the lateral spinothalamic tract. In the lateral spinothalamic tract the fibres are laminated, with sacral fibres lateral and cervical fibres medial.

Why is "Sacral fibres run in the dorsal columns" not the answer?

The dorsal columns carry touch and vibration, not pain and temperature.

Why is "Sacral fibres lie most medially in the lateral spinothalamic tract" not the answer?

Sacral fibres are not medial; the medial fibres are cervical.

Why is "Sacral fibres decussate lower than other fibres and are compressed fi…" not the answer?

All spinothalamic fibres cross within a segment or two of entry, so crossing level does not explain it.

What concept does this question test?

It tests Neuroanatomy: spinothalamic lamination in Anatomy. An external compression hits the lateral fibres first, so loss ascends from the sacral level. An intramedullary lesion such as syringomyelia affects medial fibres first and may spare the sacral region.

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 Neuroanatomy: spinothalamic lamination?

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.