Neuroanatomy: false-localising hemiparesis: 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: false-localising hemiparesis

Sample item

A 45-year-old man with a large right-sided acute subdural haematoma has a dilated, unreactive right pupil but weakness of the right arm and leg. Which mechanism best explains the right-sided weakness?

ALeft cerebral peduncle pressed on the tentorial edgeBRight cerebral peduncle compressed by the uncusCCompression of the pyramidal decussation at the foramen magnumDRight internal capsule infarction from stretched perforators

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

The answer and the reasoning

A. Left cerebral peduncle pressed on the tentorial edge

A large right-sided mass pushes the midbrain across the midline, so the opposite (left) cerebral peduncle is pressed against the free edge of the tentorium, producing a Kernohan notch. Corticospinal fibres in the left peduncle supply the right side of the body, so the hemiparesis lies on the same side as the mass and is falsely localising.

Option by option

A. Left cerebral peduncle pressed on the tentorial edge (correct)
Correct. The left peduncle carries corticospinal fibres to the right limbs, so its compression gives right-sided weakness.
B. Right cerebral peduncle compressed by the uncus
Compression of the right peduncle would cause the expected left-sided weakness, not weakness on the right.
C. Compression of the pyramidal decussation at the foramen magnum
Pyramidal decussation compression belongs to tonsillar herniation and would not produce this isolated right-sided pattern.
D. Right internal capsule infarction from stretched perforators
No infarct is described, and a right internal capsule infarct would paralyse the left side of the body.

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.

Questions about this Anatomy question

What is the answer to this Anatomy question?

The best answer is A, Left cerebral peduncle pressed on the tentorial edge. A large right-sided mass pushes the midbrain across the midline, so the opposite (left) cerebral peduncle is pressed against the free edge of the tentorium, producing a Kernohan notch.

Why is "Right cerebral peduncle compressed by the uncus" not the answer?

Compression of the right peduncle would cause the expected left-sided weakness, not weakness on the right.

Why is "Compression of the pyramidal decussation at the foramen magnum" not the answer?

Pyramidal decussation compression belongs to tonsillar herniation and would not produce this isolated right-sided pattern.

Why is "Right internal capsule infarction from stretched perforators" not the answer?

No infarct is described, and a right internal capsule infarct would paralyse the left side of the body.

What concept does this question test?

It tests Neuroanatomy: false-localising hemiparesis in Anatomy. Corticospinal fibres in the left peduncle supply the right side of the body, so the hemiparesis lies on the same side as the mass and is falsely localising.

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: false-localising hemiparesis?

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.