Cranial nerves: 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 · Cranial nerves: oculomotor nerve compression

Sample item

A 60-year-old woman has a sudden severe headache, a drooping right eyelid, an eye turned down and out, and a dilated unreactive right pupil. Why is the pupil involved early in compression of the nerve by a posterior communicating artery aneurysm?

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

The answer and the reasoning

B. The parasympathetic fibres run on the outer surface of the nerve and are compressed first

The pupillomotor parasympathetic fibres from the Edinger-Westphal nucleus lie superficially in the oculomotor nerve, so external compression, as by an expanding aneurysm at the junction of the posterior communicating and internal carotid arteries, affects them early. In microvascular ischaemia, as in diabetes, the central fibres are affected first and the pupil is often spared. Pupil involvement is therefore an urgent warning of a compressive cause.

Option by option

  • A. The parasympathetic fibres run in the centre of the nerve, close to the vasa nervorum

    Central fibres are typically affected by ischaemia, which spares the pupil.

  • B. The parasympathetic fibres run on the outer surface of the nerve and are compressed first (correct)

    Correct. The superficial position makes them vulnerable to extrinsic compression.

  • C. The sympathetic fibres to the dilator pupillae run within the nerve

    The dilator pupillae is supplied by sympathetic fibres, which do not travel within the oculomotor nerve.

  • D. The pupillary fibres travel in the ophthalmic division of the trigeminal nerve

    The pupil constrictor fibres travel in the oculomotor nerve, not the trigeminal nerve.

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, The parasympathetic fibres run on the outer surface of the nerve and are compressed first. The pupillomotor parasympathetic fibres from the Edinger-Westphal nucleus lie superficially in the oculomotor nerve, so external compression, as by an expanding aneurysm at the junction of the posterior communicating and internal carotid arteries, affects them early.

Why is "The parasympathetic fibres run in the centre of the nerve, close to t…" not the answer?

Central fibres are typically affected by ischaemia, which spares the pupil.

Why is "The sympathetic fibres to the dilator pupillae run within the nerve" not the answer?

The dilator pupillae is supplied by sympathetic fibres, which do not travel within the oculomotor nerve.

Why is "The pupillary fibres travel in the ophthalmic division of the trigemi…" not the answer?

The pupil constrictor fibres travel in the oculomotor nerve, not the trigeminal nerve.

What concept does this question test?

It tests Cranial nerves: oculomotor nerve compression in Anatomy. In microvascular ischaemia, as in diabetes, the central fibres are affected first and the pupil is often spared. Pupil involvement is therefore an urgent warning of a compressive cause.

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 Cranial nerves: 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.