Neuroanatomy: afferent pupillary defect: 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: afferent pupillary defect

Sample item

In a patient with optic neuritis the swinging flashlight test is performed. When the light is moved from the left eye to the right eye, both pupils dilate; when moved back to the left eye, both constrict. Which lesion is present?

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

The answer and the reasoning

A. Right optic nerve (afferent limb)

A relative afferent pupillary defect arises when one optic nerve conducts the light signal less well. Light in the affected (right) eye causes a weaker drive to both Edinger-Westphal nuclei than light in the healthy eye, so both pupils dilate when light is swung to it. Efferent lesions give an unreactive pupil in one eye but the consensual response of the other eye is preserved.

Option by option

  • A. Right optic nerve (afferent limb) (correct)

    A right optic nerve lesion causes paradoxical dilatation when the light is swung to the right eye.

  • B. Left Edinger-Westphal nucleus

    Each Edinger-Westphal nucleus receives input from both eyes, so a unilateral lesion causes an efferent defect without a relative afferent defect.

  • C. Right oculomotor nerve

    A right oculomotor lesion would leave the right pupil dilated regardless of which eye is illuminated.

  • D. Left optic nerve

    A left optic nerve lesion would produce dilatation when the light is swung to the left eye.

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.

More Anatomy questions

Questions about this Anatomy question

What is the answer to this Anatomy question?

The best answer is A, Right optic nerve (afferent limb). A relative afferent pupillary defect arises when one optic nerve conducts the light signal less well.

Why is "Left Edinger-Westphal nucleus" not the answer?

Each Edinger-Westphal nucleus receives input from both eyes, so a unilateral lesion causes an efferent defect without a relative afferent defect.

Why is "Right oculomotor nerve" not the answer?

A right oculomotor lesion would leave the right pupil dilated regardless of which eye is illuminated.

Why is "Left optic nerve" not the answer?

A left optic nerve lesion would produce dilatation when the light is swung to the left eye.

What concept does this question test?

It tests Neuroanatomy: afferent pupillary defect in Anatomy. Light in the affected (right) eye causes a weaker drive to both Edinger-Westphal nuclei than light in the healthy eye, so both pupils dilate when light is swung to it. Efferent lesions give an unreactive pupil in one eye but the consensual response of the other eye is preserved.

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: afferent pupillary defect?

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.