**Core Concept**
Iron deficiency anemia is a condition characterized by a decrease in the production of hemoglobin due to insufficient iron availability. The body's stores of iron are depleted, affecting the production of red blood cells. To correct this deficiency, iron therapy is administered to replenish the body's iron stores.

**Why the Correct Answer is Right**
The response to iron therapy in a patient with iron deficiency anemia is denoted by an increase in serum ferritin levels. Serum ferritin is a protein that stores iron in the body, and its levels are directly proportional to the body's iron stores. When iron therapy is initiated, the levels of serum ferritin increase as the body's iron stores are replenished. This increase in serum ferritin is an indicator of the effectiveness of the iron therapy. The increase in serum ferritin is accompanied by an increase in hemoglobin and red blood cell production, which helps to alleviate the symptoms of iron deficiency anemia.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because an increase in serum transferrin receptors is not a direct indicator of the response to iron therapy. Although transferrin receptors do increase in iron deficiency anemia, their levels decrease as the body's iron stores are replenished.
**Option B:** This is incorrect because an increase in mean corpuscular volume (MCV) is not a direct indicator of the response to iron therapy. While MCV may increase as hemoglobin production increases, it is not a specific marker of iron therapy effectiveness.
**Option C:** This is incorrect because an increase in reticulocyte count is a sign of bone marrow response, but not a direct indicator of the response to iron therapy. The increase in reticulocyte count may occur before the increase in serum ferritin and hemoglobin.

**Clinical Pearl / High-Yield Fact**
To assess the response to iron therapy, clinicians should monitor serum ferritin levels regularly. A sustained increase in serum ferritin levels is a reliable indicator of the effectiveness of iron therapy and the replenishment of the body's iron stores.

**Correct Answer: D. Increased serum ferritin levels.**