## **Core Concept**
Sickle cell anemia is a genetic disorder that affects hemoglobin production. It is characterized by the production of abnormal hemoglobin, known as hemoglobin S (HbS), which leads to the deformation of red blood cells under certain conditions. The primary defect in sickle cell anemia involves a mutation in the HBB gene that codes for the beta-globin subunit of hemoglobin.

## **Why the Correct Answer is Right**
The correct answer, , involves a point mutation in the HBB gene that codes for the beta-globin subunit of hemoglobin. This mutation results in the substitution of glutamic acid with valine at position 6 of the beta-globin chain (Glu6Val). This change leads to the production of abnormal hemoglobin S (HbS), which has a tendency to polymerize under low oxygen conditions, causing red blood cells to assume a sickle shape.

## **Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while genomic instability can lead to various genetic disorders, it is not the specific primary defect leading to sickle cell anemia.
* **Option B:** This option is incorrect as it does not specifically relate to the known genetic defect causing sickle cell anemia.
* **Option C:** This option is incorrect because it refers to a different type of genetic defect and is not relevant to the pathogenesis of sickle cell anemia.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that sickle cell anemia is an autosomal recessive disorder, meaning that a person must inherit two copies of the mutated gene (one from each parent) to express the disease. Carriers, who have one normal and one mutated gene, typically have a milder condition known as sickle cell trait.

## **Correct Answer:** . Substitution of glutamic acid with valine at position 6 of the beta-globin chain.