This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nervous system, Subject name: Physiology
During the dark phase of visual cycle, which form of vitamin A combines with opsin to make Rhodopsin 
Select and explain. 
A. All trans-Retinaldehyde, B. All trans-retinol, C. 11-cis-Retinaldehyde, D. 11-cis-Retinol
C: 11-cis-Retinaldehyde
Answer: C: 11-cis-Retinaldehyde 
Explanation: Rhodopsin is the photosensitive visual pigment present in the discs of the rod outer segments. It consists of a protein opsin and a carotenoid called retinal or retinene (aldehyde of vitamin A). The light absorbed by the rhodopsin conves its 11-cis-retinal into all-trans-retinal. This light-induced isomerization of 11-cis-retinal into all-trans-retinal occurs through formation of many intermediates which exist for a transient period. In the retina, under constant light stimulation, a steady state must exist under which the rate at which they are regenerated. This equilibrium between the photodecomposition and regeneration of visual pigments is referred to as visual cycle. Ref: guyton and hall textbook of medical physiology 12 edition page number: 742,743,744