During the dark phase of visual cycle, which form of vitamin A combines with opsin to make Rhodopsin:
**Core Concept**
The dark phase of the visual cycle, also known as the bleaching phase, occurs when light is absent. During this phase, rhodopsin, the light-sensitive pigment in the retina, undergoes a series of chemical reactions to regenerate 11-cis retinal, a form of vitamin A.
**Why the Correct Answer is Right**
In the dark phase, rhodopsin is broken down into opsin and 11-cis retinal. The 11-cis retinal then combines with opsin to reform rhodopsin. This process involves the enzyme retinol isomerase, which catalyzes the isomerization of all-trans retinal to 11-cis retinal. The 11-cis retinal is a form of vitamin A that combines with opsin to make rhodopsin, which is essential for light detection in the retina.
**Why Each Wrong Option is Incorrect**
**Option A:** All-trans retinal is the form of vitamin A that is produced during the bleaching phase, but it does not combine with opsin to make rhodopsin. Instead, it undergoes isomerization to 11-cis retinal.
**Option B:** 9-cis retinal is a form of vitamin A that is involved in the visual cycle, but it is not the form that combines with opsin to make rhodopsin.
**Option D:** No option is given for this letter, and it is not relevant.
**Clinical Pearl / High-Yield Fact**
The visual cycle is a complex process that involves the conversion of light into electrical signals in the retina. Rhodopsin is a crucial component of this process, and its regeneration is essential for maintaining normal visual function. A deficiency in the enzymes involved in the visual cycle, such as retinol isomerase, can lead to night blindness.
**Correct Answer: C. 11-cis retinal.**