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
Which of the following tests is most sensitive for the detection of iron depletion in pregnancy ? 
Select and explain. 
A. Serum iron, B. Serum ferritin, C. Serum transferrin, D. Serum iron binding capacity
B: Serum ferritin
Answer: B: Serum ferritin 
Explanation: Serum ferritin is the most sensitive test as it correlates best with iron stores and is the first test to become abnormal in case of iron deficiency.
Remember: 

Storage form of iron : – Ferritin
Transport form of iron – Transferrin

As per CDC serum ferritin less than 15 mg/l confirms iron deficiency anemia.