Neuroanatomy: oculomotor nerve compression: 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: oculomotor nerve compression

Sample item

A 50-year-old woman has a sudden severe headache. The right eye is deviated down and out with ptosis, and the right pupil is dilated and unreactive. Which explanation best accounts for early pupillary involvement in an aneurysm at the posterior communicating artery?

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

The answer and the reasoning

B. Parasympathetic fibres run superficially in the oculomotor nerve and are compressed first

The parasympathetic pupilloconstrictor fibres lie in the outer (peripheral) part of the oculomotor nerve, close to the posterior communicating artery, so an expanding aneurysm compresses them early and the pupil dilates. In diabetic ischaemic third nerve palsy the central fibres are affected and the peripheral pupillary fibres are spared. The Edinger-Westphal nucleus lies in the midbrain, away from the aneurysm.

Option by option

  • A. Parasympathetic fibres run in the core of the nerve and are damaged first by ischaemia

    Core placement is wrong, and the mechanism describes pupil-sparing ischaemic palsy.

  • B. Parasympathetic fibres run superficially in the oculomotor nerve and are compressed first (correct)

    Superficial placement of the pupillary fibres makes the pupil an early sign of compression.

  • C. The Edinger-Westphal nucleus is crushed directly by the aneurysm

    The nucleus is in the midbrain and is not near the aneurysm, which acts on the nerve in the subarachnoid space.

  • D. Sympathetic fibres to the dilator pupillae are paralysed

    Sympathetic denervation causes a small pupil and not a dilated one.

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, Parasympathetic fibres run superficially in the oculomotor nerve and are compressed first. The parasympathetic pupilloconstrictor fibres lie in the outer (peripheral) part of the oculomotor nerve, close to the posterior communicating artery, so an expanding aneurysm compresses them early and the pupil dilates.

Why is "Parasympathetic fibres run in the core of the nerve and are damaged f…" not the answer?

Core placement is wrong, and the mechanism describes pupil-sparing ischaemic palsy.

Why is "The Edinger-Westphal nucleus is crushed directly by the aneurysm" not the answer?

The nucleus is in the midbrain and is not near the aneurysm, which acts on the nerve in the subarachnoid space.

Why is "Sympathetic fibres to the dilator pupillae are paralysed" not the answer?

Sympathetic denervation causes a small pupil and not a dilated one.

What concept does this question test?

It tests Neuroanatomy: oculomotor nerve compression in Anatomy. In diabetic ischaemic third nerve palsy the central fibres are affected and the peripheral pupillary fibres are spared. The Edinger-Westphal nucleus lies in the midbrain, away from the aneurysm.

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: oculomotor nerve compression?

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.