Megaloblastic anemia caused by vitamin B-12 deficiency due to impaired absorption. The impaired absorption of vitamin B-12 is secondary to atrophic gastritis and loss of gastric parietal cells or caused by defective production of intrinsic factor (a carrier protein) by the gastric mucosa. Megaloblastic anemia caused by vitamin B-12 deficiency due to impaired absorption. The impaired absorption of vitamin B-12 is secondary to atrophic gastritis and loss of gastric parietal cells.