**Core Concept**
Sickle cell anaemia is a genetic disorder characterized by abnormal haemoglobin (HbS) that causes red blood cells to become misshapen and break down prematurely, leading to chronic anaemia, vaso-occlusive crises, and increased risk of infections. In severe cases, patients may require blood transfusions to improve oxygen delivery and reduce the risk of complications.

**Why the Correct Answer is Right**
Blood transfusions are indicated in sickle cell anaemia when there is a significant drop in haemoglobin levels, acute chest syndrome, stroke, or severe vaso-occlusive crisis. The transfused red blood cells are primarily exchanged for the patient's sickled red blood cells, reducing the amount of HbS and improving the overall haemoglobin level. This can help alleviate symptoms, reduce the frequency of complications, and improve the patient's quality of life.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because blood transfusions are generally not indicated for mild anaemia in sickle cell anaemia patients, as they may lead to iron overload and other complications.

**Option B:** This option is incorrect because while hydroxyurea (hydroxycarbamide) is used to reduce the frequency of painful crises and acute chest syndrome in sickle cell anaemia patients, it is not a substitute for blood transfusions in severe cases.

**Option C:** This option is incorrect because pain management is an important aspect of sickle cell anaemia care, but it is not a primary indication for blood transfusions.

**Clinical Pearl / High-Yield Fact**
In patients with sickle cell anaemia, blood transfusions can lead to iron overload, which can be managed with regular iron chelation therapy to prevent long-term complications.

**Correct Answer:** B.