**Core Concept**
The patient's symptoms suggest an immune-mediated glomerulonephritis, specifically membranous nephropathy, which can be a complication of hepatitis B infection. This condition involves the deposition of immune complexes containing hepatitis B surface antigen (HBsAg) in the glomerular basement membrane, leading to glomerular damage.

**Why the Correct Answer is Right**
The deposition of immune complexes activates the complement system, leading to the formation of membrane attack complexes (MACs) that damage the glomerular basement membrane. This damage disrupts the glomerular filtration barrier, allowing protein and red blood cells to leak into the urine. The presence of red cell casts indicates glomerular origin of the hematuria.

**Why Each Wrong Option is Incorrect**

* **Option A:** Focal segmental glomerulosclerosis (FSGS) is a primary glomerular disease characterized by scarring of glomeruli, but it is not typically associated with hepatitis B infection.
* **Option B:** Minimal change disease is a common cause of nephrotic syndrome in children, but it is not typically associated with hepatitis B infection and does not typically present with hematuria and red cell casts.
* **Option C:** Rapidly progressive glomerulonephritis (RPGN) is a severe form of glomerulonephritis that can be caused by various conditions, including immune complex-mediated disease, but it is not specifically associated with hepatitis B infection.

**Clinical Pearl / High-Yield Fact**
Immune complex deposition in the glomerular basement membrane can lead to various forms of glomerulonephritis, including membranous nephropathy, which is a common cause of nephrotic syndrome in adults.

**Correct Answer:** C.