**Core Concept**
Sickle cell anaemia is a genetic disorder caused by a mutation in the HBB gene, which encodes the beta-globin subunit of haemoglobin. This mutation leads to the production of abnormal haemoglobin (HbS), characterized by a substitution of glutamic acid with valine at position 6 of the beta-globin chain.

**Why the Correct Answer is Right**
The correct answer is due to a point mutation in the HBB gene, resulting in the substitution of glutamic acid (GAG codon) with valine (GTG codon) at position 6 of the beta-globin chain. This mutation causes the polymerization of deoxygenated HbS, leading to the formation of rigid, sickle-shaped red blood cells that are prone to vaso-occlusion and tissue damage. The HBB gene is located on chromosome 11, and the mutation is inherited in an autosomal recessive manner.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify the underlying genetic cause of sickle cell anaemia.
* **Option B:** This option is incorrect because it mentions a different genetic disorder, thalassemia, which is caused by mutations in the HBA1/HBA2 or HBB genes, but is not the primary cause of sickle cell anaemia.
* **Option C:** This option is incorrect because it mentions a viral infection, which is not the cause of sickle cell anaemia.

**Clinical Pearl / High-Yield Fact**
Sickle cell anaemia is a classic example of a genetic disorder that is prevalent in areas with high malaria transmission, as the sickle cell trait provides some protection against malaria. This is known as heterozygous advantage.

**Correct Answer: D. A point mutation in the HBB gene resulting in the substitution of glutamic acid with valine at position 6 of the beta-globin chain.**