This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Vitamins and Minerals, Subject name: Biochemistry
[{'content': 'Which of the following is the major circulating form of Vitamin D? \nSelect and explain. \nA. 25-(OH) D3, B. 1, 25-(OH)2 D3, C. 24, 25-(OH)2 D3, D. 1, 24-(OH)2 D3', 'role': 'user'}, {'content': "Answer: A \nExplanation: 25-(OH) cholecalciferol is the major circulating form of vitamin D because of the half-life of 3-weeks 1,25-dihydroxy vitamin D3 a.k.a. calcitriol is the biologically active form vitamin D. But, it's half life is low. In clinical laboratory, we measure 25-(OH) cholecalciferol.", 'role': 'assistant'}]