Neuroanatomy: lumbosacral root lesions: 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: lumbosacral root lesions

Sample item

A 35-year-old man with sciatica has weak plantar flexion, numbness along the lateral border of the foot and an absent ankle jerk. A disc prolapse is compressing which nerve root?

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

The answer and the reasoning

B. S1

The ankle jerk depends mainly on S1, with plantar flexion power and sensation over the lateral foot. A posterolateral L5-S1 disc prolapse compresses the traversing S1 root, whereas L5 compression gives weak dorsiflexion and a foot drop.

Option by option

  • A. L4

    L4 compression weakens knee extension and reduces the knee jerk, with sensory loss over the medial leg.

  • B. S1 (correct)

    Correct. Absent ankle jerk, weak plantar flexion and lateral foot numbness all localise to this root.

  • C. L5

    L5 compression causes weak dorsiflexion and extensor hallucis longus weakness, with the ankle jerk preserved.

  • D. L3

    L3 compression affects hip flexion and knee extension, with sensory loss over the anterior thigh.

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, S1. The ankle jerk depends mainly on S1, with plantar flexion power and sensation over the lateral foot.

Why is "L4" not the answer?

L4 compression weakens knee extension and reduces the knee jerk, with sensory loss over the medial leg.

Why is "L5" not the answer?

L5 compression causes weak dorsiflexion and extensor hallucis longus weakness, with the ankle jerk preserved.

Why is "L3" not the answer?

L3 compression affects hip flexion and knee extension, with sensory loss over the anterior thigh.

What concept does this question test?

It tests Neuroanatomy: lumbosacral root lesions in Anatomy. A posterolateral L5-S1 disc prolapse compresses the traversing S1 root, whereas L5 compression gives weak dorsiflexion and a foot drop.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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: lumbosacral root lesions?

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.