This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
An old man having complaint of anorexia, weakness and paraesthesia. On investigation, his haemoglobin was 5.8g% and peripheral smear showed macrocytes and neutrophils having hypersegmented nuclei. His tendon reflexes were also sluggish. Endoscopy revealed atrophic gastritis. Deficiency of which of the following factor can lead to this situation 
Select and explain. 
A. Folic acid, B. Riboflavin, C. VitaminB12, D. None
C: VitaminB12
Answer: C: VitaminB12 
Explanation: 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.