A patient complains of difficulty seeing at night after sunset. The visual cycle requires continuous regeneration of 11-cis-retinal from all-trans-retinal in the retinal pigment epithelium. Which form of vitamin A is the chromophore bound to opsin in rhodopsin?

• A Retinoic acid
• B Retinol
• C 11-cis-retinal ✓
• D Retinyl palmitate

Explanation

In photoreceptors, 11-cis-retinal is the aldehyde form covalently bound to opsin to form rhodopsin. Light isomerizes it to all-trans-retinal, which must be converted back to 11-cis in the RPE via the visual cycle. Retinoic acid regulates gene transcription through nuclear receptors and cannot serve as the visual chromophore because its beta-ionone ring geometry cannot be photoisomerized in this way.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.