Which protein is the primary iron storage protein whose serum level reflects marrow storage iron?

• A. Transferrin
• B. Ferritin
• C. Hemoglobin
• D. Hemosiderin

Answer: (B)

The question tests which protein best reflects the amount of iron stored in the body, particularly in marrow. Ferritin is the main intracellular iron storage protein, with a small portion circulating in serum. Because serum ferritin generally tracks the total body iron stores, it serves as the best indicator of marrow iron stores. Transferrin, by contrast, is the transport protein that carries iron in the blood; its levels relate to iron transport capacity rather than stored iron. Hemoglobin uses iron to form heme but its level depends on production and destruction of red cells, not directly on how much iron is stored. Hemosiderin is a storage form that appears under certain conditions but is not a reliable marker of current stores. Keep in mind ferritin can be influenced by inflammation, which can elevate it independent of stores, so clinical context matters.