Vision biochemistry — Which trace mineral is a critical component or cofactor for enzymes (such as retinol dehydrogenases) that activate vitamin A to its visual form (retinal/retinaldehyde) in the eye?

Introduction / Context:
Night vision depends on the rapid interconversion of vitamin A compounds within the retina. A trace mineral cofactor is required by key enzymes that convert retinol to retinal (retinaldehyde), the chromophore that couples to opsins to form rhodopsin for low-light vision. This question checks recognition of the trace element that enables vitamin A activation in ocular tissues.

Given Data / Assumptions:

• The eye requires vitamin A in its aldehyde form (retinal/11-cis-retinal) to form functional photopigments.
• Retinol dehydrogenases and related enzymes catalyze retinoid interconversions.
• Trace minerals serve as enzyme cofactors; deficiency impairs enzyme activity.

Concept / Approach:
Zinc acts as a structural and catalytic cofactor for multiple retinol dehydrogenases and supports hepatic synthesis of retinol-binding protein, enabling transport of vitamin A. Insufficient zinc compromises the conversion of retinol to retinal and reduces rhodopsin regeneration, clinically manifesting as impaired dark adaptation or night blindness even when vitamin A intake is adequate.

Step-by-Step Solution:

Verification / Alternative check:
Clinical and experimental data show delayed dark adaptation in zinc deficiency, improved by zinc repletion when vitamin A status is normal. Serum retinol-binding protein also depends on adequate zinc, reinforcing this relationship.

Why Other Options Are Wrong:

• Iron: central to hemoglobin and many oxidoreductases, but not the specific retinol dehydrogenase cofactor.
• Iodine: required for thyroid hormones, not for ocular vitamin A activation.
• Chromium: potentiates insulin action; no direct role in retinoid activation.
• Selenium: cofactor for glutathione peroxidases; antioxidant, not the retinol-activating cofactor.

Common Pitfalls:
Assuming night blindness always equals vitamin A deficiency. Zinc deficiency can mimic it by impairing vitamin A utilization rather than intake per se.

Final Answer:
Zinc.