(D) Vitamin $B_{12}$ (cyanocobalamin) is known as Castle's extrinsic factor. It is called 'extrinsic' because it must be obtained from the diet. In th

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(D) Vitamin $B_{12}$ (cyanocobalamin) is known as Castle's extrinsic factor. It is called 'extrinsic' because it must be obtained from the diet. In the stomach, it binds to the 'intrinsic factor' (secreted by parietal cells) to facilitate its absorption in the ileum of the small intestine.