This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
[{'content': 'Which of the following is the most sensitive indicator of iron depletion during pregnancy? \nSelect and explain. \nA. Serum iron, B. Serum ferritin, C. Serum transferrin, D. Serum iron binding capacity', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Serum ferritin levels is the first line test of iron status in the body, because it is an early indicator of iron store depletion. Serum ferritin less than 15mcg/L is suggestive of absent body iron stores. Ref: Textbook of Therapeutics: Drug and Disease Management By Richard A. Helms, 8th Edition, Page 781 ; Clinical Laboratory Pearls By Steven L. Jones, Pages 67-8 ; Textbook of Obstetrics By D.C.Dutta, 6th Edition, Pages 265-6', 'role': 'assistant'}]