This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Central Nervous system, Subject name: Pharmacology
A patient on Phenytoin therapy develops depression, for which he was prescribed tricyclic anti-Depressants. He now complains of lassitude and his Hb reading is 8gm/dl, the next step in the management of this patient is: 
Select and explain. 
A. Chest X ray, B. MCV should be estimated, C. GGT should be estimated, D. None of the above
B: MCV should be estimated
Answer: B: MCV should be estimated 
Explanation: phenytoin can induce folate deficient megaloblastic anemia that can be diagnozed by measuring MCV. REF: KD TRIPATHI 8TH ED.