Vitamin B-12 deficiency

In medicine, vitamin B-12 deficiency, is a "nutritional condition produced by a deficiency of Vitamin B-12 in the diet, characterized by megaloblastic anemia. Since vitamin B 12 is not  present in plants, humans have obtained their supply from animal  products, from multivitamin supplements in the form of pills, and as  additives to food preparations. A wide variety of neuropsychiatric  abnormalities is also seen in vitamin B 12 deficiency and appears to be due to an undefined defect involving myelin synthesis."

Manifestations include pernicious anemia and subacute combined degeneration.

Diagnosis
The MCV is not accurate.