A type of anaemia in which red blood cells grow abnormally large and immature because defective DNA synthesis prevents normal cell division. It most commonly results from deficiency of vitamin B12 or folate, both essential for DNA production. Symptoms include fatigue, pallor, sore tongue, and neurological changes in B12 deficiency. Diagnosis relies on blood films showing oversized red cells and on measuring vitamin levels. Treatment with the missing vitamin produces rapid improvement, but correct diagnosis matters: giving folate alone to someone with B12 deficiency can mask the anaemia while allowing nerve damage to progress. Dietary causes include poor intake and malabsorption.