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What is An old man, Om Prakash presented with anorexia, weakness, and paraesthesia. On further investigation, his hemoglobin came out to be 5.8 g% and the peripheral smear showed the presence of macrocytes and neutrophils having hypersegmented nuclei. His tendon reflexes also were sluggish. Endoscopy revealed atrophic gastritis. Deficiency of which of the following factors can lead to such a clinical situation? and explain in detail?
Diagnosis of the patient is pernicious anemia. Factors favouring this diagnosis are:

Megaloblastic anemia.
Demyelination (decreased tendon reflexes)
Atrophic gastritis

So, he would require B12 supplementation.