Which of the following is the active form of vitamin A that functions as a cofactor in the visual cycle?

Explanation

## Active Form of Vitamin A in Vision **Key Point:** 11-cis-retinal is the light-sensitive chromophore that binds to opsin in the retina and undergoes isomerization during the visual cycle. ### The Visual Cycle 1. **11-cis-retinal** (active form) binds to opsin → forms rhodopsin 2. Light absorption → isomerization to **all-trans-retinal** 3. Conformational change → neural signal transmission 4. All-trans-retinal is reduced to all-trans-retinol 5. Retinol is re-isomerized back to 11-cis-retinal (regeneration) ### Vitamin A Forms and Their Roles | Form | Role | Location | | --- | --- | --- | | **11-cis-retinal** | Visual chromophore; light-sensitive | Retinal photoreceptors | | All-trans-retinol | Transport form; storage | Blood, liver | | Retinyl palmitate | Storage ester | Liver stellate cells | | Retinoic acid | Gene regulation; transcription factor | Nuclear receptors | **High-Yield:** Only 11-cis-retinal can function in the visual cycle because its bent geometry is essential for opsin binding and light-induced isomerization. All-trans-retinal cannot bind opsin effectively. **Clinical Pearl:** Deficiency of vitamin A → impaired regeneration of 11-cis-retinal → night blindness (earliest sign of vitamin A deficiency).