**Core Concept**
Steroid-resistant nephrotic syndrome (SRNS) is a condition where patients do not respond to high doses of corticosteroids, such as methylprednisolone. Focal segmental glomerulosclerosis (FSGS) is a common cause of SRNS, characterized by scarring and fibrosis in the glomeruli of the kidneys. The pathophysiology involves the disruption of the glomerular filtration barrier, leading to massive proteinuria.

**Why the Correct Answer is Right**
In steroid-resistant FSGS, the next line of treatment typically involves immunosuppressive therapy to reduce proteinuria and slow disease progression. Cyclosporine is a calcineurin inhibitor that acts by inhibiting the activation of T-lymphocytes, leading to a decrease in the production of pro-inflammatory cytokines and a reduction in proteinuria. Cyclosporine has been shown to be effective in reducing proteinuria and improving renal function in patients with SRNS due to FSGS.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because mycophenolate mofetil is an immunosuppressive agent that is typically used in conjunction with other treatments, such as calcineurin inhibitors, but is not the first-line treatment for SRNS due to FSGS.
**Option B:** This option is incorrect because rituximab is a monoclonal antibody that targets CD20-positive B cells and is used in the treatment of autoimmune diseases, but its efficacy in SRNS due to FSGS is not well-established.
**Option C:** This option is incorrect because sirolimus is another calcineurin inhibitor, but it is not typically used as a first-line treatment for SRNS due to FSGS, and its use is often limited by its side effects.

**Clinical Pearl / High-Yield Fact**
Cyclosporine is a common first-line treatment for SRNS due to FSGS, but its use requires close monitoring of kidney function and blood pressure, as well as regular blood tests to check for adverse effects such as nephrotoxicity and hypertension.

**Correct Answer:** C. Cyclosporine