This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Hematology, Subject name: Pharmacology
A 40 year old man has megaloblastic anemia and early signs of neurological abnormality. The drug most probably required is: 
Select and explain. 
A. Folic acid, B. Iron sulphate, C. Erythropoietin, D. Vitamin B12
D: Vitamin B12
Answer: D: Vitamin B12 
Explanation: Deficiency of vitamin B12 results in megaloblastic anemia and demyelination. It can cause subacute combined degeneration of spinal cord and peripheral neuritis.