💡 Explanation (Answer: B)

Under hypoxic conditions, the corneal epithelium shifts from aerobic respiration to anaerobic glycolysis, leading to an accumulation of lactate in the stroma. This osmotic buildup draws water directly into the corneal stroma, resulting in swelling, disruption of collagen spacing, and loss of transparency.

💡 Explanation (Answer: B)

Rod cells contain the highly light-sensitive photopigment rhodopsin, allowing them to detect single photons and mediate scotopic vision in dimly lit environments. In contrast, cone cells require much higher light intensities and are responsible for photopic color vision and high-acuity central vision.

💡 Explanation (Answer: C)

The meibomian glands located within the tarsal plates secrete meibum, which constitutes the outer lipid layer of the precorneal tear film. This hydrophobic layer plays a critical role in lubricating the ocular surface and preventing the rapid evaporation of the underlying aqueous layer.

💡 Explanation (Answer: C)

Glutathione is the primary antioxidant in the crystalline lens and actively maintains vitamin C and vitamin E in their active, reduced states. This continuous reduction pathway is vital for neutralizing free radicals and protecting structural lens proteins from oxidative damage.

💡 Explanation (Answer: D)

The vitreous humor contains a concentration of ascorbate that is up to forty times higher than that found in blood plasma. This elevated level is maintained by active transport systems in the ciliary body to neutralize reactive oxygen species and shield the lens and retina from oxidative stress.

💡 Explanation (Answer: C)

Exposure to light triggers a phototransduction cascade that closes cyclic nucleotide-gated sodium channels in the outer segment of the rod cell. This reduction in sodium influx causes the resting membrane potential to drop, leading to hyperpolarization and a decrease in glutamate release at the synapse.

💡 Explanation (Answer: C)

Vitamin A serves as the essential organic precursor for 11-cis-retinal, which combines with opsin proteins to synthesize the light-sensitive pigment rhodopsin within rod outer segments. A systemic deficiency directly compromises rod phototransduction, impairing visual adaptation in dimly lit environments and causing night blindness.

💡 Explanation (Answer: C)

During accommodation, parasympathetic stimulation causes the ciliary muscle to contract, which slackens the zonules of Zinn attached to the lens capsule. This release of tension allows the elastic crystalline lens to naturally become more spherical, increasing its anterior curvature and refractive power for near vision.

💡 Explanation (Answer: D)

Maximal dark adaptation is a biphasic process that requires approximately 20 to 30 minutes to achieve full sensitivity. While cone photoreceptors adapt rapidly within the first 5 to 10 minutes, the slower regeneration of rod photopigments requires a longer period to reach peak low-light sensitivity.

💡 Explanation (Answer: C)

Rhodopsin is a specialized G-protein-coupled receptor photopigment embedded within the disc membranes of the rod outer segments. This pigment is highly sensitive to photons and initiates the visual phototransduction cascade when exposed to even low levels of light.