Special senses: pupillary light reflex pathway: practice question

A Physiology 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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Physiology · Special senses: pupillary light reflex pathway

Sample item

After a head injury, a man's right pupil is 7 mm and does not constrict when light is shone into either eye. The left pupil is 3 mm and constricts normally to light in either eye, and vision is intact. Which structure is damaged?

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

The answer and the reasoning

C. The right oculomotor nerve carrying parasympathetic efferent fibres

The light reflex runs from the retina through the optic nerve to the pretectal nuclei, then to both Edinger-Westphal nuclei and out through the oculomotor nerves to the sphincter pupillae. The right pupil fails to constrict even when the right and left eyes are lit, so the afferent side is intact and the right efferent limb is damaged, as in uncal herniation. The intact left efferent limb gives the normal consensual response.

Option by option

  • A. The left oculomotor nerve

    A left oculomotor lesion would affect the left pupil, which constricts normally.

  • B. The right optic nerve

    A right optic nerve lesion would spare the right pupil's response to light shone in the left eye.

  • C. The right oculomotor nerve carrying parasympathetic efferent fibres (correct)

    Correct. An efferent defect makes one pupil dilated and unreactive to light in both eyes.

  • D. The pretectal nuclei on both sides

    Bilateral pretectal damage would affect both pupils equally.

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 Physiology question

What is the answer to this Physiology question?

The best answer is C, The right oculomotor nerve carrying parasympathetic efferent fibres. The light reflex runs from the retina through the optic nerve to the pretectal nuclei, then to both Edinger-Westphal nuclei and out through the oculomotor nerves to the sphincter pupillae.

Why is "The left oculomotor nerve" not the answer?

A left oculomotor lesion would affect the left pupil, which constricts normally.

Why is "The right optic nerve" not the answer?

A right optic nerve lesion would spare the right pupil's response to light shone in the left eye.

Why is "The pretectal nuclei on both sides" not the answer?

Bilateral pretectal damage would affect both pupils equally.

What concept does this question test?

It tests Special senses: pupillary light reflex pathway in Physiology. The right pupil fails to constrict even when the right and left eyes are lit, so the afferent side is intact and the right efferent limb is damaged, as in uncal herniation. The intact left efferent limb gives the normal consensual response.

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 Special senses: pupillary light reflex pathway?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.