Neuro-ophthalmology and pupils: apraclonidine test: practice question

A Ophthalmology 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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Ophthalmology · Neuro-ophthalmology and pupils: apraclonidine test

Sample item

A man with left ptosis and a smaller left pupil is given 1 percent apraclonidine in both eyes. After 40 minutes the left pupil dilates and the left lid lifts, while the right pupil shows no change. What explains this response?

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

The answer and the reasoning

A. Denervation supersensitivity of alpha-1 receptors on the dilator muscle

In Horner syndrome the iris dilator becomes supersensitive to alpha-agonists. Apraclonidine has weak alpha-1 activity, which is enough to dilate the denervated pupil and reverse the anisocoria, while the normal pupil barely responds. Cocaine, in contrast, depends on intact nerve terminals.

Option by option

  • A. Denervation supersensitivity of alpha-1 receptors on the dilator muscle (correct)

    Correct. Supersensitive alpha-1 receptors on a denervated dilator respond to weak apraclonidine.

  • B. Blockage of norepinephrine reuptake at the nerve terminal

    Blocking reuptake is the mechanism of cocaine, which fails to dilate a Horner pupil.

  • C. Release of stored norepinephrine from intact nerve endings

    Release of stored norepinephrine is the action of hydroxyamphetamine and needs intact terminals.

  • D. Paralysis of the iris sphincter muscle

    Sphincter paralysis would give a dilated pupil at baseline, not a small pupil.

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 Ophthalmology question

What is the answer to this Ophthalmology question?

The best answer is A, Denervation supersensitivity of alpha-1 receptors on the dilator muscle. In Horner syndrome the iris dilator becomes supersensitive to alpha-agonists.

Why is "Blockage of norepinephrine reuptake at the nerve terminal" not the answer?

Blocking reuptake is the mechanism of cocaine, which fails to dilate a Horner pupil.

Why is "Release of stored norepinephrine from intact nerve endings" not the answer?

Release of stored norepinephrine is the action of hydroxyamphetamine and needs intact terminals.

Why is "Paralysis of the iris sphincter muscle" not the answer?

Sphincter paralysis would give a dilated pupil at baseline, not a small pupil.

What concept does this question test?

It tests Neuro-ophthalmology and pupils: apraclonidine test in Ophthalmology. Apraclonidine has weak alpha-1 activity, which is enough to dilate the denervated pupil and reverse the anisocoria, while the normal pupil barely responds. Cocaine, in contrast, depends on intact nerve terminals.

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 Neuro-ophthalmology and pupils: apraclonidine test?

Group the red eye by what to examine and what the pupil does. Then use the flashcards and the other Ophthalmology 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.