**Core Concept**
The patient presents with signs of anemia, including pallor, lethargy, and irritability. The laboratory findings indicate a microcytic, hypochromic anemia with low hemoglobin (Hb) and mean corpuscular volume (MCV) levels. The presence of target cells and normoblasts on the peripheral blood smear suggests an inherited anemia.

**Why the Correct Answer is Right**
The patient's clinical presentation and laboratory findings are consistent with beta-thalassemia major, a genetic disorder characterized by a mutation in the HBB gene that codes for the beta-globin subunit of hemoglobin. The expansion of erythroid marrow in the skull is a hallmark of this condition, as the bone marrow attempts to compensate for the ineffective erythropoiesis caused by the genetic mutation. The normal osmotic fragility test rules out hereditary spherocytosis, a different cause of hemolytic anemia.

**Why Each Wrong Option is Incorrect**
**Option A:** Sickle cell anemia would typically present with a different set of laboratory findings, including a positive sickle test and evidence of hemolysis.

**Option B:** Hereditary spherocytosis is ruled out by the normal osmotic fragility test, which would be abnormal in this condition.

**Option C:** Glucose-6-phosphate dehydrogenase (G6PD) deficiency would typically present with episodes of hemolysis triggered by certain medications or infections.

**Clinical Pearl / High-Yield Fact**
Beta-thalassemia major is a genetic disorder that requires lifelong transfusion therapy and iron chelation to manage the consequences of chronic anemia and iron overload.

**Correct Answer:** C.