**Core Concept**
Sickle cell anaemia is a genetic disorder caused by a mutation in the HBB gene that encodes for the beta-globin subunit of hemoglobin. This mutation leads to the production of abnormal hemoglobin, known as hemoglobin S (HbS), which causes red blood cells to become misshapen and prone to sickling under low oxygen conditions.

**Why the Correct Answer is Right**
The defect in sickle cell anaemia arises from a point mutation in the HBB gene, where a single nucleotide is substituted at position 6 of the beta-globin gene. This substitution changes glutamic acid to valine at position 6 (Glu6Val), resulting in the production of HbS. This point mutation is an example of a base substitution, where a single nucleotide is replaced with a different nucleotide. This substitution leads to a change in the amino acid sequence of the beta-globin protein, resulting in the abnormal hemoglobin associated with sickle cell anaemia.

**Why Each Wrong Option is Incorrect**
**Option A:** A base insertion in DNA would involve the addition of an extra nucleotide, which is not the mechanism underlying sickle cell anaemia.
**Option B:** A base deletion in DNA would involve the removal of a nucleotide, which is also not the mechanism underlying sickle cell anaemia.
**Option D:** While none of the above options accurately describe the mechanism underlying sickle cell anaemia, option C is the correct answer, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
It's worth noting that sickle cell anaemia is an autosomal recessive disorder, meaning that an individual must inherit two copies of the mutated gene (one from each parent) to express the disease. This makes it essential for individuals with a family history of sickle cell anaemia to undergo genetic testing.

**✓ Correct Answer: C. A base substitution in DNA**