**Question:** The shape of RBC is biconcave due to

A. increase in surface area
B. presence of spectrin protein
C. presence of ankyrin protein
D. less cytoplasm

**Core Concept:** The biconcave shape of red blood cells (RBCs) is essential for their function and efficiency in the circulatory system. This unique shape increases the surface area, enabling efficient exchange of gases (oxygen and carbon dioxide) between RBCs and the surrounding tissues. The biconcave shape also allows RBCs to easily pass through narrow capillaries.

**Why the Correct Answer is Right:** The correct answer, B, refers to spectrin, a protein that forms a network underneath the cell membrane of RBCs. This spectrin network helps maintain the biconcave shape by providing structural support and stability to the RBCs.

**Why Each Wrong Option is Incorrect:**

A. Option A, increase in surface area, is partially correct but not the main reason for the biconcave shape. While an increase in surface area is a result of the biconcave shape, it is the presence of spectrin that actually causes the shape.

C. Option C, ankyrin protein, is not directly responsible for the biconcave shape of RBCs. Ankyrin is involved in anchoring band 3 protein and spectrin to maintain the cell membrane integrity and shape, but it is spectrin that results in the biconcave shape.

D. Option D, less cytoplasm, is unrelated to the biconcave shape of RBCs. The cytoplasm's composition and volume play a role in determining the cell's mechanical properties, but the main reason for the biconcave shape is the presence of spectrin and its structural role in maintaining the cell membrane integrity and shape.

**Clinical Pearl:** The biconcave shape of RBCs is primarily determined by the presence of spectrin and its role in maintaining the cell membrane integrity and shape. This unique shape allows RBCs to efficiently transport oxygen and carbon dioxide between the lungs and tissues, as well as facilitate gas exchange in the lungs.

In summary, the biconcave shape of RBCs is primarily determined by the presence of spectrin, a protein that forms a network underneath the cell membrane, providing structural support and stability to the RBCs. Spectrin, along with other proteins, maintains the cell membrane integrity and shape, enabling efficient gas exchange and transportation of oxygen and carbon dioxide in the circulatory system.