A 34-year-old man presents with a 5-month history of weakness and fatigue. There is no history of drug or alcohol abuse. A CBC shows megaloblastic anemia and a normal reticulocyte count. Further laboratory studies reveal vitamin B12 deficiency Anemia in this patient is most likely caused by which of the following?

Autoimmune gastritis refers to chronic, diffuse inflammatory disease of the stomach that is restricted to the body and fundus and is associated with other autoimmune phenomena. This disorder typically features diffuse atrophic gastritis, antibodies to parietal cells and the intrinsic factor, and increased serum gastrin due to G-cell hyperplasia. Immunologic destruction of parietal cells and antibody targeting of intrinsic factor interfere with intestinal absorption of vitamin B12 . As a result, all lineages of bone marrow precursors show asynchronous maturation between the nucleus and cytoplasm (megaloblastic cells), and the peripheral blood displays megaloblastic anemia. Megaloblastic anemia that is caused by malabsorption of vitamin B12 , occasioned by a deficiency of the intrinsic factor, is referred to as "pernicious anemia." The other choices are not causes of pernicious anemia.Diagnosis: Autoimmune atrophic gastritis, pernicious anemia