**Core Concept**
Macrocytic anaemia in children is characterized by an increase in mean corpuscular volume (MCV) due to a reduction in the production of red blood cells (RBCs) or an increase in the size of RBCs. This type of anaemia is often caused by a deficiency in vitamin B12 or folate, which are essential for DNA synthesis and RBC production.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of macrocytic anaemia. Vitamin B12 and folate are crucial for the synthesis of DNA and the production of RBCs. A deficiency in these nutrients leads to an accumulation of homocysteine and methylmalonic acid, which in turn causes an increase in MCV. This is because the body tries to compensate for the lack of these nutrients by producing larger RBCs.

**Why Each Wrong Option is Incorrect**
**Option A:** Thalassemia major is a genetic disorder that affects the production of the beta-globin chains of haemoglobin, leading to microcytic anaemia, not macrocytic anaemia.
**Option B:** Congenital hypoplastic anemia is a rare genetic disorder that affects the production of RBCs, leading to macrocytic anaemia.
**Option C:** Fanconi anaemia is a genetic disorder that affects the production of RBCs and leads to macrocytic anaemia.
**Option D:** Sickle cell anaemia is a genetic disorder that affects the production of the beta-globin chains of haemoglobin, leading to microcytic anaemia, not macrocytic anaemia.

**Clinical Pearl / High-Yield Fact**
Remember that macrocytic anaemia is often caused by a deficiency in vitamin B12 or folate, and a careful history and physical examination are crucial in diagnosing these deficiencies.

**Correct Answer: D. Sickle cell anaemia is a genetic disorder that affects the production of the beta-globin chains of haemoglobin, leading to microcytic anaemia, not macrocytic anaemia.**