Given below are two statements: one is labeled Assertion (A) and other is labeled Reason (R).

Assertion (A): At low oxygen tension, the biconcave disc like RBC change to elongated sickle structure in sickle cell anaemia.

Reason (R): The mutant haemoglobin molecule with abnormal $\mathrm{\beta }$ chain undergoes polymerisation under low oxygen tension causing the change in the shape of the RBC from biconcave disc to elongated sickle like structure.
 

In the light of above statements, choose the correct answer from the options given below.

The given Assertion (A) and Reason (R) are both correct, and Reason (R) explains Assertion (A).

Sickle cell anemia is caused by a mutation in the hemoglobin gene that results in the production of abnormal hemoglobin, known as hemoglobin S (HbS). Under low oxygen tension, HbS molecules can polymerise and form long, rigid structures within red blood cells (RBCs). This polymerisation causes the normally biconcave-shaped RBCs to change into an elongated, sickle-like shape. These sickle-shaped RBCs can get stuck in blood vessels, leading to various complications associated with sickle-cell anemia.

So, Assertion (A) is true because RBCs in sickle-cell anemia change from a biconcave shape to an elongated sickle shape, and Reason (R) provides a valid explanation for this phenomenon.