Neuroanatomy: oculomotor nerve: 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

Sample item

After a head injury a patient develops an enlarging extradural haematoma, and the ipsilateral pupil becomes fixed and dilated before eye movements are lost. Why are the pupillomotor fibres affected first?

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

The answer and the reasoning

C. They lie superficially on the oculomotor nerve and are compressed against the tentorial edge

Parasympathetic fibres run on the outer surface of the oculomotor nerve and are compressed first when the uncus herniates over the tentorial edge. In diabetic ischaemic third nerve palsy the central core of the nerve is deprived of blood, so the pupil is typically spared. This difference helps separate compressive from medical causes.

Option by option

  • A. They lie centrally in the nerve and are the most sensitive to ischaemia

    Central placement of the fibres would spare them in compression and describes pupil sparing in diabetes.

  • B. The Edinger-Westphal nucleus is crushed directly by the haematoma

    The Edinger-Westphal nucleus lies in the midbrain and is not crushed directly in early uncal herniation.

  • C. They lie superficially on the oculomotor nerve and are compressed against the tentorial edge (correct)

    Correct. Superficial parasympathetic fibres are compressed first by uncal herniation.

  • D. Sympathetic dilator fibres become overactive on the same side

    Sympathetic fibres from the superior cervical ganglion are not hyperactive, they are not involved in this mechanism.

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 C, They lie superficially on the oculomotor nerve and are compressed against the tentorial edge. Parasympathetic fibres run on the outer surface of the oculomotor nerve and are compressed first when the uncus herniates over the tentorial edge.

Why is "They lie centrally in the nerve and are the most sensitive to ischaem…" not the answer?

Central placement of the fibres would spare them in compression and describes pupil sparing in diabetes.

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

The Edinger-Westphal nucleus lies in the midbrain and is not crushed directly in early uncal herniation.

Why is "Sympathetic dilator fibres become overactive on the same side" not the answer?

Sympathetic fibres from the superior cervical ganglion are not hyperactive, they are not involved in this mechanism.

What concept does this question test?

It tests Neuroanatomy: oculomotor nerve in Anatomy. In diabetic ischaemic third nerve palsy the central core of the nerve is deprived of blood, so the pupil is typically spared. This difference helps separate compressive from medical causes.

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?

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.