Lower limb: dermatomes: 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 · Lower limb: dermatomes

Sample item

A 40-year-old man has back pain with numbness over the medial malleolus, a diminished knee jerk and weakness of ankle dorsiflexion. Which nerve root is most likely compressed?

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

The answer and the reasoning

D. L4

The L4 dermatome includes the medial leg and the medial malleolus, and L4 contributes to the knee jerk (L3 and L4) and to tibialis anterior. L5 supplies the dorsum of the foot and great toe, and S1 supplies the lateral foot and sole and mediates the ankle jerk. Mapping sensory loss to a dermatome localises the root level.

Option by option

  • A. S2

    S2 supplies the posterior thigh and upper calf, not the medial malleolus.

  • B. L5

    L5 supplies the dorsum of the foot and the great toe and preserves the knee jerk.

  • C. S1

    S1 supplies the lateral foot and its loss reduces the ankle jerk.

  • D. L4 (correct)

    Correct. Medial malleolar sensory loss with a reduced knee jerk fits L4.

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 D, L4. The L4 dermatome includes the medial leg and the medial malleolus, and L4 contributes to the knee jerk (L3 and L4) and to tibialis anterior.

Why is "S2" not the answer?

S2 supplies the posterior thigh and upper calf, not the medial malleolus.

Why is "L5" not the answer?

L5 supplies the dorsum of the foot and the great toe and preserves the knee jerk.

Why is "S1" not the answer?

S1 supplies the lateral foot and its loss reduces the ankle jerk.

What concept does this question test?

It tests Lower limb: dermatomes in Anatomy. L5 supplies the dorsum of the foot and great toe, and S1 supplies the lateral foot and sole and mediates the ankle jerk. Mapping sensory loss to a dermatome localises the root level.

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 Lower limb: dermatomes?

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.