This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: G.I.T, Subject name: Physiology
Earliest change in iron deficiency anaemia is 
Select and explain. 
A. Decreased serum iron, B. Decreased serum ferritin, C. Decreased TIBC, D. Decreased hemoglobin
B: Decreased serum ferritin
Answer: B: Decreased serum ferritin 
Explanation: Ferritin is a blood cell protein that contains iron. A ferritin test helps your doctor understand how much iron your body is storing. If a ferritin test reveals that your blood ferritin level is lower than normal, it indicates your body's iron stores are low and you have iron deficiency Ref:guyton and hall textbook of medical physiology 12 edition page number:118,119,120