**Core Concept**
The Finnish type of congenital nephrotic syndrome is a rare genetic disorder characterized by the loss of foot processes of podocytes, leading to massive proteinuria. It is caused by mutations in a specific gene that encodes a crucial protein involved in maintaining the integrity of the glomerular filtration barrier.

**Why the Correct Answer is Right**
The Finnish type of congenital nephrotic syndrome is caused by mutations in the NPHS1 gene, which encodes the protein nephrin. Nephrin is a transmembrane protein that plays a key role in the formation and maintenance of the slit diaphragm, a critical structure in the glomerular filtration barrier. Mutations in the NPHS1 gene lead to the loss of nephrin from the slit diaphragm, resulting in the collapse of the filtration barrier and massive proteinuria.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because mutations in the NPHS2 gene, which encodes podocin, are associated with a different form of congenital nephrotic syndrome, known as steroid-resistant nephrotic syndrome.
* **Option B:** This option is incorrect because mutations in the WT1 gene are associated with Wilms tumor and other forms of nephropathy, but not the Finnish type of congenital nephrotic syndrome.
* **Option C:** This option is incorrect because mutations in the COL4A3 gene are associated with Alport syndrome, a different type of kidney disease characterized by glomerulonephritis and hearing loss.

**Clinical Pearl / High-Yield Fact**
The Finnish type of congenital nephrotic syndrome is a rare but severe form of kidney disease that is usually diagnosed in utero or in early childhood. Early recognition and management are crucial to prevent complications and improve outcomes.

**Correct Answer:** C.