**Core Concept**
Iron deficiency anemia is characterized by a decrease in serum iron levels, but the first laboratory indices to change are actually related to the body's iron stores. Serum ferritin reflects the body's stored iron, and its levels decrease early in iron deficiency anemia.

**Why the Correct Answer is Right**
Serum ferritin is a protein that stores iron in the body, and its levels decrease early in iron deficiency anemia. This is because ferritin is bound to iron, and when iron levels drop, ferritin is released into the bloodstream, causing a decrease in serum ferritin levels. This change occurs before any significant decrease in serum iron or hemoglobin levels. **Ferritin is a sensitive indicator of iron stores**, and its levels can decrease even before serum iron levels drop.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum iron levels decrease later in iron deficiency anemia, after serum ferritin levels have already dropped.
**Option B:** Total iron binding capacity (TIBC) actually increases in iron deficiency anemia, not decreases, as the body produces more transferrin to bind to the remaining iron.
**Option D:** Serum hemoglobin concentration decreases later in iron deficiency anemia, as the lack of iron affects hemoglobin production.

**Clinical Pearl / High-Yield Fact**
Remember that **serum ferritin is a good indicator of iron stores**, and its levels can decrease even before serum iron levels drop. This is why ferritin is often used as a screening test for iron deficiency anemia.

**✓ Correct Answer: C. S. Ferritin**