**Core Concept**
Hereditary spherocytosis is a genetic disorder characterized by the production of abnormal, sphere-shaped red blood cells. This condition arises due to defects in the proteins that make up the red blood cell membrane, leading to premature destruction of the cells in the spleen.

**Why the Correct Answer is Right**
The most common cause of hereditary spherocytosis is a deficiency of spectrin, a protein that maintains the shape and structure of the red blood cell membrane. Spectrin deficiency leads to a loss of membrane stability and an increase in the osmotic fragility of red blood cells, resulting in their premature destruction. Other proteins, such as ankyrin, band 3, and protein 4.2, can also be involved in hereditary spherocytosis, but spectrin deficiency is the most common cause.

**Why Each Wrong Option is Incorrect**
**Option A:** Glycophorin A deficiency is associated with a different condition known as M-N blood group, not hereditary spherocytosis.
**Option B:** Band 3 deficiency can be a cause of hereditary spherocytosis, but it is less common than spectrin deficiency.
**Option C:** Protein 4.2 deficiency is associated with hereditary spherocytosis, but it is a rare cause of the condition.

**Clinical Pearl / High-Yield Fact**
Hereditary spherocytosis is an autosomal dominant disorder, meaning that a single copy of the mutated gene is sufficient to cause the condition.

**Correct Answer: A. Spectrin deficiency.**