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
[{'content': 'Most sensitive marker for iron deficiency is - \nSelect and explain. \nA. Hemoglobin, B. Ferritin, C. Transferring saturation, D. Serum iron', 'role': 'user'}, {'content': "Answer: B \nExplanation: 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.", 'role': 'assistant'}]