pernicious anemia

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A severe anemia most often affecting older adults, caused by failure of the stomach to absorb vitamin B₁₂ and marked by a decrease in number and an increase in the size of red blood cells, by gastrointestinal disturbances, and by lesions of the spinal cord.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun pathology A severe form of anemia caused by vitamin B12 insufficiency.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a chronic progressive anemia of older adults; thought to result from a lack of intrinsic factor (a substance secreted by the stomach that is responsible for the absorption of vitamin B12)