Oculomotor nerve: uncal herniation: 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 · Oculomotor nerve: uncal herniation

Sample item

A patient with an expanding right subdural haematoma has a dilated, sluggish right pupil, and later ptosis with a down-and-out eye. Why does the pupil become involved first?

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

The answer and the reasoning

A. The parasympathetic pupillomotor fibres run on the outer surface of the oculomotor nerve and are compressed first by the herniating uncus

The oculomotor nerve crosses the free edge of the tentorium, close to the uncus of the temporal lobe. Its pupillary parasympathetic fibres are superficial and are the first to be compressed, so the pupil dilates before the extraocular muscles weaken. Ptosis and the down-and-out position follow with progressive compression.

Option by option

  • A. The parasympathetic pupillomotor fibres run on the outer surface of the oculomotor nerve and are compressed first by the herniating uncus (correct)

    Correct. The superficial position of the parasympathetic fibres makes them vulnerable to external compression.

  • B. The Edinger-Westphal nucleus is infarcted before any other part of the midbrain

    Infarction of the nucleus is a later feature of brainstem haemorrhage and would not explain the early sign.

  • C. The abducens nerve is the first to be stretched by the herniating uncus

    The abducens nerve supplies lateral rectus and its involvement does not explain the dilated pupil.

  • D. The sympathetic fibres to the dilator pupillae run in the core of the oculomotor nerve and are compressed first

    The dilator pupillae receives sympathetic fibres from the carotid plexus and not through the oculomotor 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 A, The parasympathetic pupillomotor fibres run on the outer surface of the oculomotor nerve and are compressed first by the herniating uncus. The oculomotor nerve crosses the free edge of the tentorium, close to the uncus of the temporal lobe.

Why is "The Edinger-Westphal nucleus is infarcted before any other part of th…" not the answer?

Infarction of the nucleus is a later feature of brainstem haemorrhage and would not explain the early sign.

Why is "The abducens nerve is the first to be stretched by the herniating unc…" not the answer?

The abducens nerve supplies lateral rectus and its involvement does not explain the dilated pupil.

Why is "The sympathetic fibres to the dilator pupillae run in the core of the…" not the answer?

The dilator pupillae receives sympathetic fibres from the carotid plexus and not through the oculomotor nerve.

What concept does this question test?

It tests Oculomotor nerve: uncal herniation in Anatomy. Its pupillary parasympathetic fibres are superficial and are the first to be compressed, so the pupil dilates before the extraocular muscles weaken. Ptosis and the down-and-out position follow with progressive compression.

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 Oculomotor nerve: uncal herniation?

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.