Absorption of vitamin B12 in humans requires "P" glycoprotein secreted from "Q". The correct choice of P and Q is
A. P=Extrinsic factor and Q=Stomach
B. P=Intrinsic factor and Q=Stomach
C. P=Intrinsic factor and Q=Small intestine
D. P=Exopolysaccharide and Q=Small intestine

Hint: Vitamin B12 is a water-soluble vitamin. It plays an important role in the metabolism of every cell in the human body. Vitamin B12 is a cofactor in DNA synthesis. It also plays an important role in myelin synthesis and thus, helps in the functioning of the nervous system.

Complete answer: Vitamin B12 is a large and complex vitamin. It has to be broken down into a smaller substance and then has to be absorbed. An intrinsic factor, which is a glycoprotein is necessary for B12 vitamin absorption. It is secreted by the parietal of the stomach cell. If intrinsic factor fails to bind to vitamin B12, pernicious anaemia occurs. Absorption of vitamin B12 occurs in two ways:
First is the absorption of Vitamin B12 by a specific intestinal mechanism using intrinsic factor through which one or two micrograms can be absorbed every few hours. The second step is by the process of passive diffusion.
Binding of the intrinsic factor protects vitamin B12 from catabolism by intestinal bacteria. Certain intrinsic factor receptors are present on the ileal mucosa which bind the complex, allowing vitamin B12 to be absorbed into portal blood.
Hence, the correct answer is option B.

Additional information: Stomach is a muscular sac-like organ that stores the bolus. The wall of the stomach secreted digestive enzymes like trypsin that helps in the digestion of proteins.

Note: Deficiency of B12 causes irreversible damage, mainly to the brain and the nervous system. If the level of B12 decreases slightly than normal value, it shows symptoms like fatigue, pale skin, poor memory, depression, and headache.