## **Core Concept**
Sickle cell anaemia is a genetic disorder that affects the structure and function of **haemoglobin**, a protein in red blood cells that carries oxygen to the body's tissues. The disorder is characterized by the production of abnormal haemoglobin, known as **haemoglobin S (HbS)**. This abnormality affects a specific property of haemoglobin.

## **Why the Correct Answer is Right**
In sickle cell anaemia, the property of haemoglobin that is affected is its **shape and flexibility**. Normally, haemoglobin is flexible and can change shape easily, allowing red blood cells to pass through small blood vessels. However, HbS has a mutation in the beta-globin subunit that causes it to polymerize under low oxygen conditions, leading to the formation of rigid, sickle-shaped red blood cells. This rigidity affects the cells' ability to navigate through narrow capillaries, leading to vaso-occlusive crises and other complications.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while sickle cell anaemia does affect the oxygen-carrying capacity of blood indirectly by reducing the number of functional red blood cells, the primary issue is not the oxygen-carrying capacity itself but the physical properties of the haemoglobin.
- **Option B:** This option might relate to the affinity of haemoglobin for oxygen, but the primary defect in sickle cell anaemia is not related to oxygen affinity; it's related to the polymerization of deoxyhaemoglobin S.
- **Option C:** This could potentially relate to the solubility of haemoglobin, which is indeed affected. The polymerization of deoxyhaemoglobin S reduces its solubility, leading to the formation of fibres within red blood cells. However, the most direct and commonly emphasized effect is on the shape of red blood cells.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that sickle cell anaemia results from a point mutation in the HBB gene that codes for the beta subunit of haemoglobin, leading to **glutamic acid to valine substitution** at position 6 (Glu6Val). This change causes haemoglobin to polymerize under low oxygen conditions, resulting in sickle-shaped red blood cells.

## **Correct Answer:** . Shape and flexibility