This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Blood, Subject name: Medicine
Most sensitive marker for iron deficiency is - 
Select and explain. 
A. Hemoglobin, B. Ferritin, C. Transferring saturation, D. Serum iron
B: Ferritin
Answer: B: Ferritin 
Explanation: Ans. is 'b' i.e., Ferritin o Of all the serum tests for iron deficiency anemia, the serum ferritin determination is the most useful and a low serum ferritin invariably signifies iron deficiency.