**Core Concept**
Thalassemia major, a severe form of beta-thalassemia, is characterized by significant hemolysis and bone marrow expansion. This leads to characteristic radiological findings, including a 'hair on end' appearance on skeletal radiographs.

**Why the Correct Answer is Right**
The 'hair on end' appearance in thalassemia major is due to the expansion of the bone marrow, which causes thinning and cortical thinning of the bones, particularly in the skull and long bones. This results in a characteristic radiographic appearance, where the trabeculae of the bone are arranged perpendicular to the cortical surface, resembling hair. This is a consequence of the bone marrow's attempt to compensate for the inadequate production of hemoglobin, leading to increased erythropoiesis.

**Why Each Wrong Option is Incorrect**
**Option A:** Sickle cell anemia can cause bone changes, such as avascular necrosis and osteonecrosis, but it is not characteristically associated with the 'hair on end' appearance.
**Option B:** G 6PD deficiency is a hemolytic anemia, but it does not cause significant bone marrow expansion or the characteristic radiographic findings seen in thalassemia major.
**Option C:** Hereditary spherocytosis can cause anemia and splenomegaly, but it is not typically associated with the 'hair on end' appearance on skeletal radiographs.

**Clinical Pearl / High-Yield Fact**
The 'hair on end' appearance is a classic radiological sign of thalassemia major and should be considered in the context of a patient's anemia, bone pain, and other systemic symptoms.

✓ Correct Answer: A. Thalasemia major