This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Special Senses, Subject name: Physiology
[{'content': 'Which substance is most likely to increase in the rods of the retina when the light is turned on? \nSelect and explain. \nA. Cyclic adenosine monophosphate (cAMP), B. Cyclic guanosine monophosphate (cGMP), C. Metarhodopsin II, D. Rhodopsin', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Photons activate rhodopsin to become metarhodopsin II in the rods of the retina, which means that exposure to light decreases the concentration of rhodopsin and increases the concentration of metarhodopsin II, also called activated rhodopsin. Metarhodopsin II leads to a decrease in cGMP through a series of biochemical reactions cAMP levels are unchanged. Vitamin A levels are not likely to change with exposure to light', 'role': 'assistant'}]