**Core Concept**
Iron deficiency anemia is a condition characterized by a decrease in the body's iron stores, leading to a reduction in the production of hemoglobin, a protein in red blood cells that carries oxygen to tissues. The earliest changes in iron deficiency anemia occur at the cellular level, affecting the production and function of red blood cells.

**Why the Correct Answer is Right**
The earliest change in iron deficiency anemia is a decrease in serum ferritin levels. Serum ferritin reflects the body's iron stores, and a decrease in ferritin levels indicates that the body's iron stores are depleted. This is because ferritin is a protein that stores iron in the body, and when iron levels are low, ferritin levels also decrease. This decrease in ferritin levels occurs before any changes in hemoglobin or red blood cell count, making it the earliest indicator of iron deficiency anemia.

**Why Each Wrong Option is Incorrect**
* **Option A:** A decrease in mean corpuscular volume (MCV) is a sign of iron deficiency anemia, but it occurs later than a decrease in serum ferritin levels.
* **Option B:** An increase in total iron-binding capacity (TIBC) is a sign of iron deficiency anemia, but it occurs later than a decrease in serum ferritin levels.
* **Option C:** A decrease in red blood cell count is a sign of iron deficiency anemia, but it occurs later than a decrease in serum ferritin levels.

**Clinical Pearl / High-Yield Fact**
In iron deficiency anemia, a decrease in serum ferritin levels is the earliest and most sensitive indicator of iron deficiency. This is because ferritin levels reflect the body's iron stores, and a decrease in ferritin levels indicates that the body's iron stores are depleted, even before any changes in hemoglobin or red blood cell count.

**Correct Answer:** D. Decrease in serum ferritin levels.