This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Active form of vitamin Li is 
Select and explain. 
A. Cholecalciferol, B. 24, 25 (OH)2 vit-D, C. 1,25(OH) 2 vit-D, D. 25-OH vit-D
C: 1,25(OH) 2 vit-D
Answer: C: 1,25(OH) 2 vit-D 
Explanation: 1,25(011) 2 vit-D (Ref: Harper 26/e, p 484-485; Harrison 16/e, p 2246: Chatterjee Shinde Biochemistry 6/e, p 151] Synthesis and metabolism of Vitamin D Calcidiol may undergo 24-hydroxylation to yield an inactive metabolite, 24, 25-dihydroxy vit. D mediated by vit. D 24 - hydroxylase. 1, 25 (01-1)2D is the major inducer of vit-D-24 hydroxylase, thus promotes its own inactivation, thereby limiting its biologic effects.