**Core Concept**
Alport's syndrome is a genetic disorder characterized by glomerulonephritis, end-stage kidney disease, and hearing loss. It is primarily inherited in an X-linked pattern, affecting males more frequently than females.

**Why the Correct Answer is Right**
Alport's syndrome is caused by mutations in the COL4A5 gene, which encodes for the alpha-5 chain of type IV collagen. This protein is crucial for the formation of the glomerular basement membrane in the kidneys. The mutation leads to the production of abnormal type IV collagen, causing the characteristic histopathological changes and clinical manifestations. The COL4A5 gene is located on the X chromosome, explaining the X-linked inheritance pattern.

**Why Each Wrong Option is Incorrect**
**Option B:** COL4B6 gene is not associated with Alport's syndrome. The correct gene associated with Alport's syndrome is COL4A5, not COL4B6.
**Option C:** COL3A5 gene is a distractor, as it is not related to Alport's syndrome. The correct gene is COL4A5, not COL3A5.
**Option D:** COL4A3 gene is involved in Alport's syndrome, but it is an autosomal form, not the X-linked form. The X-linked form is due to mutations in the COL4A5 gene.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Alport's syndrome is typically inherited in an X-linked pattern, with males being more frequently affected than females. The disease is characterized by glomerulonephritis, end-stage kidney disease, and hearing loss.

**✓ Correct Answer: A. COL4A5 gene**