This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Physiology
[{'content': 'Site of 25-hydroxylation of cholecalciferol \nSelect and explain. \nA. Kidney, B. Skin, C. Liver, D. Lung', 'role': 'user'}, {'content': 'Answer: C \nExplanation: 7-Dehydrocholesterol is the precursor of vitamin D3. Within the epidermal layer of skin, 7-Dehydrocholesterol undergoes an electrocyclic reaction as a result of UVB radiation, resulting in the opening of the vitamin precursor B-ring through a conrotatory pathway. Following this, the previtamin D3 undergoes a antarafacial sigmatropic rearrangement and therein finally isomerizes to form vitamin D3. Cholecalciferol is then hydroxylated in the liver to become calcifediol (25-hydroxyvitamin D3).Ref: Ganong&;s review of medical physiology; 24th edition; page no:-379', 'role': 'assistant'}]