Anatomy and physiology of the eye: first step of phototransduction: 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 · Anatomy and physiology of the eye: first step of phototransduction

Sample item

A photon is absorbed by rhodopsin in a rod outer segment. Which event occurs first?

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

The answer and the reasoning

B. Isomerisation of 11-cis retinal to all-trans retinal

Light converts 11-cis retinal to all-trans retinal, which activates opsin and the transducin-phosphodiesterase cascade. cGMP falls, the channels close, the rod hyperpolarises and glutamate release decreases.

Option by option

  • A. Depolarisation of the rod membrane

    The rod is depolarised in darkness and hyperpolarises in light.

  • B. Isomerisation of 11-cis retinal to all-trans retinal (correct)

    Correct. Photoisomerisation of retinal is the primary light-dependent step.

  • C. Increased glutamate release from the rod terminal

    Glutamate release is highest in darkness and falls on illumination.

  • D. Opening of cGMP-gated sodium channels

    The cGMP-gated channels are open in darkness and close after light is absorbed.

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 B, Isomerisation of 11-cis retinal to all-trans retinal. Light converts 11-cis retinal to all-trans retinal, which activates opsin and the transducin-phosphodiesterase cascade.

Why is "Depolarisation of the rod membrane" not the answer?

The rod is depolarised in darkness and hyperpolarises in light.

Why is "Increased glutamate release from the rod terminal" not the answer?

Glutamate release is highest in darkness and falls on illumination.

Why is "Opening of cGMP-gated sodium channels" not the answer?

The cGMP-gated channels are open in darkness and close after light is absorbed.

What concept does this question test?

It tests Anatomy and physiology of the eye: first step of phototransduction in Ophthalmology. cGMP falls, the channels close, the rod hyperpolarises and glutamate release decreases.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Anatomy and physiology of the eye: first step of phototransduction?

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.