## **Core Concept**
The question pertains to the structural defect in Hemoglobin S (HbS), which is associated with sickle cell disease. This condition arises due to a mutation in the HBB gene that codes for the beta-globin subunit of hemoglobin, leading to an abnormal hemoglobin molecule.

## **Why the Correct Answer is Right**
The correct answer, **6**, refers to the position in the beta-globin chain of hemoglobin where the mutation occurs, leading to sickle cell disease. Specifically, in HbS, glutamic acid (which is normally present at position 6 of the beta-globin chain) is replaced by valine. This substitution from a polar, negatively charged amino acid to a nonpolar one causes hemoglobin to polymerize under low oxygen conditions, resulting in the characteristic sickling of red blood cells.

## **Why Each Wrong Option is Incorrect**
- **Option A: 1** - This position does not correspond to the known mutation site for sickle cell disease.
- **Option B: 3** - Similarly, this is not the correct position for the mutation.
- **Option C: 5** - This option is also incorrect as it does not align with the established mutation site.
- **Option D: 12** - This is far from the actual site of the mutation.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that sickle cell disease is caused by a point mutation in the HBB gene, leading to the substitution of valine for glutamic acid at position **6** of the beta-globin chain. This results in HbS, which polymerizes and causes red blood cells to sickle. A classic clinical correlation is that patients with sickle cell disease are at increased risk of infections, particularly with encapsulated organisms like Streptococcus pneumoniae.

## **Correct Answer: D. 6**