Unmasking Pernicious Anemia: A Reversible Cause of Pancytopenia Due to Severe Vitamin B12 Deficiency

Vitamin B12 (cobalamin) is a vital cofactor in DNA synthesis and hematopoiesis, and its deficiency can lead to a wide spectrum of clinical manifestations, ranging from asymptomatic macrocytosis to severe hematologic and neurologic complications. While megaloblastic anemia is the most common presentation, prolonged or profound deficiency can result in pancytopenia due to ineffective hematopoiesis and intramedullary cell death. This can mimic more serious bone marrow disorders, including leukemia or aplastic anemia, potentially leading to invasive diagnostic procedures if the deficiency is not promptly recognized. Pernicious anemia is the most common cause of vitamin B12 deficiency in developed countries. It is an autoimmune condition characterized by intrinsic factor deficiency due to antibodies targeting either intrinsic factor itself or gastric parietal cells. It often presents insidiously and is frequently diagnosed late, typically after complications such as pancytopenia or irreversible neurologic damage have developed. Timely identification and treatment with parenteral vitamin B12 can lead to rapid hematologic recovery and prevention of permanent neurologic injury, highlighting the importance of considering pernicious anemia in the differential diagnosis of pancytopenia.