Which of the following parts of an eye alters the size of the pupil to regulate the amount of light entering the eye?

Appeared in: AIIMS Patna NO - 2020

Explanation

The iris is the pigmented, muscular structure that controls the diameter of the pupil.
It contains two sets of muscles: the sphincter pupillae (circular muscles) and the dilator pupillae (radial muscles).
Contraction of the sphincter pupillae, stimulated by the parasympathetic nervous system, causes the pupil to constrict (miosis) in bright light.
Contraction of the dilator pupillae, stimulated by the sympathetic nervous system, causes the pupil to dilate (mydriasis) in dim light.
This mechanism effectively regulates the amount of light that reaches the retina.

Option A: The lens is a transparent structure behind the iris that changes shape to focus light on the retina, a process called accommodation. It does not control the amount of light entering the eye.
Option B: The cornea is the transparent, dome-shaped outer layer at the front of the eye. Its primary function is to refract, or bend, light as it enters the eye. It has a fixed curvature and does not change shape to regulate light.
Option C: The retina is the light-sensitive layer of tissue at the back of the eye. It contains photoreceptor cells (rods and cones) that detect light and initiate neural signals. It receives light but does not regulate its entry.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Anatomy and physiology of the eye, specifically the mechanism of light regulation as background academic context rather than a clinical decision trigger.
A key nursing assessment is checking pupillary response (PERRLA: Pupils Equal, Round, and Reactive to Light and Accommodation). It provides vital information about neurological function.
Changes in pupil size or reactivity can indicate serious conditions such as stroke, head injury, brain tumors, or drug overdose.
What if? If a patient has received atropine eye drops for an eye exam, their pupils will be dilated (mydriasis) and will not constrict in response to light. The nurse must document this expected finding and advise the patient to wear sunglasses to protect their eyes from bright light until the drug wears off.

How to Approach the Question

First, read the question carefully to identify the key function being asked: 'alters the size of the pupil to regulate the amount of light'.
Next, analyze the options provided, which are all parts of the eye.
Recall the primary function of each anatomical part listed.
The lens is for focusing. The cornea is for refraction. The retina is for photoreception.
The iris is the muscular diaphragm that controls the pupil's size.
Match the function described in the question (regulating light by changing pupil size) with the correct anatomical part (iris).

Concept Tested & Keywords

Concept Tested: Anatomy and physiology of the eye, specifically the mechanism of light regulation.
Stem keywords: eye, alters the size, pupil, regulate light
Lead-in keywords: Which

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E6 Physiology Guyton 4SAE Part 3 pp. 141-143, 168-170

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