**Core Concept**
Steroid-resistant nephritic syndrome, also known as focal segmental glomerulosclerosis (FSGS), is a complex kidney disorder characterized by the scarring of kidney tissues. This condition often fails to respond to corticosteroid therapy, necessitating alternative treatments.

**Why the Correct Answer is Right**
The primary cause of FSGS is a mutation in the podocin gene (NPHS2) or the nephrin gene (NPHS1), which are crucial for maintaining the integrity of the glomerular filtration barrier. These mutations lead to the disruption of the slit diaphragm, resulting in proteinuria and progressive kidney damage. The podocin gene encodes a protein that plays a vital role in the structural and functional integrity of the podocytes, while the nephrin gene encodes a protein that is essential for the proper functioning of the glomerular filtration barrier.

**Why Each Wrong Option is Incorrect**
**Option A:** The WT1 gene mutation is associated with Wilms tumor, a type of kidney cancer that typically affects children.
**Option B:** The COL4A3 gene mutation is associated with Alport syndrome, a genetic disorder characterized by kidney disease, hearing loss, and eye abnormalities.
**Option C:** The ACTN4 gene mutation is associated with FSGS, but it is less common than the NPHS2 gene mutation.

**Clinical Pearl / High-Yield Fact**
FSGS is a leading cause of nephrotic syndrome in adults, and the correct diagnosis of the underlying genetic mutation can guide targeted therapy and improve patient outcomes.

**Correct Answer: C. ACTN4 gene mutation is associated with FSGS, but it is less common than the NPHS2 gene mutation.**