Pupil Reflex Involves Retinal Light Detection

Steps:

• Identify stimuli affecting pupil size: primarily light intensity changes.
• Recall pupil adjustment mechanism: iris muscles contract/relax via autonomic nerves.
• Determine receptor location: retina detects light via photoreceptors, signaling brain for reflex.
• Match to diagram: label D corresponds to retina in standard eye sections.

Why D is correct:

• Retina contains photoreceptors (rods/cones) that convert light to neural signals, initiating pupillary light reflex per neurophysiology.

Why the others are wrong:

• A (cornea): Transparent outer layer for light refraction, not stimulus detection.
• B (lens): Focuses light on retina, no sensory role in pupil control.
• C (iris): Effector muscle adjusting pupil, not the receptor for light stimuli.

Final answer: D