Severe Macrocytic Anemia Uncovering Pernicious Anemia and Early-Stage Gastric Adenocarcinoma: A Case Report.

Pernicious anemia (PA) is an autoimmune cause of vitamin B12 deficiency, often secondary to chronic atrophic gastritis, which increases the risk of gastric malignancies. Early signs, like mild macrocytosis, are frequently overlooked, especially in patients with other risk factors such as long-term metformin use. We present a case illustrating the full clinical trajectory from long-standing, unexplored macrocytosis to severe anemia, ultimately leading to the diagnosis of PA and detection of early-stage gastric adenocarcinoma on surveillance esophagogastroduodenoscopy (EGD). Informed consent was obtained from the patient to publish the case. The patient was a 59-year-old man with type 2 diabetes, treated with metformin 2 g daily for over four years, who presented with severe symptomatic macrocytic anemia after approximately one year of unexplored macrocytosis on routine laboratory testing. Evaluation revealed profound vitamin B12 deficiency with evidence of hemolysis, and serologic testing confirmed PA. Surveillance EGD identified a gastric polyp, which was resected for histopathologic evaluation. Examination of the lesion revealed a moderately differentiated adenocarcinoma with associated high-grade dysplasia and submucosal invasion. Random gastric biopsies demonstrated chronic inflammation with focal atypia and were negative for