**Core Concept**
Thalassaemia major, also known as beta-thalassaemia major, is a severe form of thalassaemia characterized by significant beta-globin chain production defect. This condition leads to ineffective erythropoiesis, severe anemia, and marked expansion of bone marrow.

**Why the Correct Answer is Right**
Thalassaemia major manifests in infancy or early childhood due to significant impairment in beta-globin chain production, leading to severe anemia. The condition is caused by mutations in the HBB gene, which codes for the beta-globin subunit of hemoglobin. Ineffective erythropoiesis results from the accumulation of abnormal beta-globin chains, which triggers oxidative stress and apoptosis in erythroid precursors. This leads to severe anemia, which requires regular blood transfusions and iron chelation therapy to prevent complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Thalassaemia minor typically presents with mild anemia and is often asymptomatic, making it an unlikely choice for the severe form of the disease.
**Option B:** Thalassaemia intermedia is a milder form of thalassaemia that may not require regular blood transfusions, unlike thalassaemia major.
**Option C:** Sickle cell disease is a different genetic disorder that affects hemoglobin production, but it is not directly related to thalassaemia major.

**Clinical Pearl / High-Yield Fact**
Thalassaemia major patients are at increased risk of developing iron overload due to frequent blood transfusions, which can lead to cardiac complications and other organ damage.

**Correct Answer: D. Infancy or early childhood.**