This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: vitamins, Subject name: Biochemistry
[{'content': '25 hydroxylation of vitamin D occurs \nSelect and explain. \nA. Skin, B. Liver, C. Kidney, D. Spleen', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Synthesis of 25-OH-D3 in Liver (Calcidiol)Vitamin D2 and/or D3 binds to specific D binding protein and is transpoed to liver.It undergoes hydroxylation at 25 position, by the enzyme 25-hydroxylase, in the endoplasmic reticulum of the mitochondria of liver cells.Coenzyme/cofactors required are: *Mg++NADPH, and *Molecular O2 A cytoplasmic factor is also required, the exact nature not known. Two enzymes, an NADPH-dependant cytochrome P450 reductase and a cytochrome P450 are involved also.25-OH-D3 (calcidiol) is the major storage form of vitamin D in liver and found in appreciable amount in circulation. The blood level of 25-OH-D3 exes feedback inhibition on the enzyme 25-hydroxylase. Reference: Chatterjea Textbook of Biochemistry pg no. 168', 'role': 'assistant'}]