Neurology: third nerve palsy with pupil involvement: practice question

A General Medicine 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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General Medicine · Neurology: third nerve palsy with pupil involvement

Sample item

A patient has sudden severe headache, ptosis and a dilated non-reactive pupil with the eye down and out. What is the most likely cause?

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

The answer and the reasoning

C. Posterior communicating artery aneurysm compressing the third nerve

Parasympathetic fibres lie on the outside of the oculomotor nerve and are affected early by compression from aneurysms, so a pupil-involving third nerve palsy needs urgent vascular imaging. Diabetic ischaemic neuropathy typically spares the pupil. Myasthenia does not affect the pupil.

Option by option

  • A. Diabetic microvascular infarct of the nerve

    Diabetic third nerve palsy is classically pupil sparing.

  • B. Myasthenia gravis

    Myasthenia spares the pupil and gives fatigable weakness.

  • C. Posterior communicating artery aneurysm compressing the third nerve (correct)

    Correct. Pupil involvement suggests compression by an aneurysm.

  • D. Horner syndrome

    Horner syndrome gives a small pupil, 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 General Medicine question

What is the answer to this General Medicine question?

The best answer is C, Posterior communicating artery aneurysm compressing the third nerve. Parasympathetic fibres lie on the outside of the oculomotor nerve and are affected early by compression from aneurysms, so a pupil-involving third nerve palsy needs urgent vascular imaging.

Why is "Diabetic microvascular infarct of the nerve" not the answer?

Diabetic third nerve palsy is classically pupil sparing.

Why is "Myasthenia gravis" not the answer?

Myasthenia spares the pupil and gives fatigable weakness.

Why is "Horner syndrome" not the answer?

Horner syndrome gives a small pupil, not a dilated one.

What concept does this question test?

It tests Neurology: third nerve palsy with pupil involvement in General Medicine. Diabetic ischaemic neuropathy typically spares the pupil. Myasthenia does not affect the pupil.

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 Neurology: third nerve palsy with pupil involvement?

For each disease know the classic presentation, the best test, and the first-line treatment. Then use the flashcards and the other General Medicine 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.