a chronic progressive anemia of older adults (occurring more frequently during the fifth and later decades, rarely before 30 years of age), due to failure of absorption of vitamin B12, usually resulting from a defect of the stomach accompanied by mucosal atrophy and associated with lack of secretion of “intrinsic” factor; characterized by numbness and tingling, weakness, and a sore smooth tongue, as well as dyspnea after slight exertion, faintness, pallor of the skin and mucous membranes, anorexia, diarrhea, loss of weight, and fever; laboratory studies usually reveal greatly decreased red blood cell counts, low levels of hemoglobin, numerous characteristically oval macrocytic erythrocytes (Colour Index greater than normal, but not truly hyperchromic), and hypochlorhydria or achlorhydria, in association with a predominant number of megaloblasts and relatively few normoblasts in the bone marrow; the leukocyte count in peripheral blood may be less than normal, with relative lymphocytosis and hypersegmented neutrophils; a low level of vitamin B12 is found in peripheral red blood cells; administration of vitamin B12 results in a characteristic reticulocyte response, relief from symptoms, and an increase in erythrocytes, provided that pernicious anemia is not complicated by another disease; the condition is not actually “pernicious,” as it was before availability of therapy with vitamin B12. At least two autosomal recessive forms are known. In one there is a defect of intrinsic factor [MIM*26100] and in the other a defective absorption of vitamin B12 from the intestine [MIM*261100].
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