**Core Concept**
The biconcave shape of red blood cells (RBCs) is a unique adaptation that provides them with increased surface area to volume ratio, allowing for more efficient gas exchange and flexibility within the microcirculation.

**Why the Correct Answer is Right**
The biconcave shape of RBCs is maintained by the mechanical properties of the cell membrane and the cytoskeletal structure. The cell membrane is composed of a lipid bilayer with embedded proteins, while the cytoskeleton is made up of a network of actin filaments and spectrin. The spectrin-actin complex provides the necessary elasticity and flexibility to the cell membrane, allowing it to withstand the compressive forces within the microcirculation. The biconcave shape also helps to reduce the cell's surface tension, making it easier for it to pass through narrow capillaries.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the biconcave shape of RBCs is not due to a lack of membrane skeleton. In fact, the membrane skeleton is essential for maintaining the cell's shape and providing mechanical stability.
**Option B:** This option is incorrect because the biconcave shape of RBCs is not solely due to the presence of hemoglobin. While hemoglobin does play a role in maintaining the cell's shape, it is not the primary factor responsible for the biconcave shape.
**Option C:** This option is incorrect because the biconcave shape of RBCs is not due to a defect in the cell membrane. The cell membrane is intact and functional in normal RBCs, and the biconcave shape is a normal adaptation.

**Clinical Pearl / High-Yield Fact**
The biconcave shape of RBCs is a critical adaptation that allows them to function effectively in the microcirculation. This shape is essential for maintaining the cell's surface area to volume ratio, which is necessary for efficient gas exchange.

**Correct Answer: C.** The biconcave shape of RBCs is maintained by the mechanical properties of the cell membrane and the cytoskeletal structure.