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
Most sensitive and specific test for diagnosis of iron deficiency is: 
Select and explain. 
A. Serum iron level, B. Serum ferritin levels, C. Serum transferrin receptor populations, D. Transferrin saturation
B: Serum ferritin levels
Answer: B: Serum ferritin levels 
Explanation: 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.