**Core Concept**
The child's presentation of hypochromic microcytic anemia suggests a disorder affecting hemoglobin production or iron metabolism. Normal levels of free Erythrocyte Protoporphyrin (FEP) help rule out disorders of heme synthesis, such as Erythropoietic Protoporphyria.

**Why the Correct Answer is Right**
The child's anemia is most likely due to Iron Deficiency Anemia (IDA), as indicated by the presence of hypochromic microcytic red blood cells. Normal FEP levels suggest that the heme synthesis pathway is intact. In IDA, the body's iron stores are depleted, impairing the production of hemoglobin. This is because iron is a crucial component of the heme molecule, which is necessary for hemoglobin formation.

**Why Each Wrong Option is Incorrect**
* **Option A:** This choice is incorrect because Sideroblastic anemia typically presents with elevated FEP levels, which is not the case in this scenario.
* **Option B:** Thalassemia major is a severe form of beta-thalassemia that presents with marked microcytosis and hypochromia, but it is not typically associated with normal FEP levels.
* **Option C:** This choice is incorrect because Erythropoietic Protoporphyria (EPP) would likely present with elevated FEP levels, which is not the case in this scenario.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that normal FEP levels can help differentiate between disorders of iron metabolism and those affecting heme synthesis. This can guide further diagnostic testing and management.

**Correct Answer:** D.