This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.V.S., Subject name: Pediatrics
[{'content': 'Most sensitive and specific test for diagnosis of iron deficiency is: \nSelect and explain. \nA. Serum iron level, B. Serum ferritin levels, C. Serum transferrin receptor populations, D. Transferrin saturation', 'role': 'user'}, {'content': "Answer: B \nExplanation: b. Serum ferritin levels(Ref: Nelson's 20/e p 2323-2325, Ghai 8/e p 334-335)Low serum ferritin level invariably indicates iron deficiency and hence it is the most specific and sensitive test for iron deficiency anemia.", 'role': 'assistant'}]