Concept: This question tests your understanding of the pupillary light reflex, a crucial mechanism for regulating the amount of light entering the eye.

Why (A) is correct: When a sudden bright light hits the eye, the pupil contracts (becomes smaller). This reflex, mediated by the parasympathetic nervous system, reduces the amount of light entering the eye, protecting the retina from overstimulation and improving visual acuity in bright conditions.

Why other options are wrong:

(B) Focus of lens changes: The lens changes focus for near or far vision (accommodation), not in response to light intensity.

(C) Vitreous humor becomes liquid like: The vitreous humor is a gel-like substance that maintains the eye's shape; its consistency does not change in response to light.

(D) Retina blood supply is cut-off: Cutting off retinal blood supply would lead to severe vision loss and is not a normal physiological response to bright light.

NEET Tip: Remember that the pupil constricts in bright light (miosis) and dilates in dim light (mydriasis). This is controlled by the iris muscles (sphincter pupillae and dilator pupillae).

Correct Answer: (A)