**Core Concept**
The management of long-standing steroid-dependent nephrotic syndrome (SDNS) in a child involves tapering and discontinuing corticosteroids to minimize the risk of complications such as Cushingoid features, hypertension, and cataracts. This requires alternative immunosuppressive therapy to maintain remission.

**Why the Correct Answer is Right**
The treatment of choice in this scenario is cyclophosphamide, which is an alkylating agent that suppresses the immune system. It is effective in inducing remission in children with SDNS who are resistant to corticosteroids. Cyclophosphamide works by causing DNA cross-linking, thereby inhibiting lymphocyte proliferation and function. This leads to a decrease in proteinuria and reduction in the need for corticosteroids.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because the treatment of choice in SDNS is not typically a calcineurin inhibitor like tacrolimus. While tacrolimus can be effective in certain cases of SDNS, it is not the first-line treatment.
**Option B:** This option might be a consideration for the treatment of SDNS, but it is not the most appropriate choice in this scenario. Mycophenolate mofetil is an immunosuppressive agent that can be used to maintain remission in SDNS, but its effectiveness compared to cyclophosphamide is less well-established.
**Option C:** This option is incorrect because rituximab is not typically used as a first-line treatment for SDNS. While rituximab can be effective in certain cases of autoimmune diseases, its use in SDNS is not well-established.

**Clinical Pearl / High-Yield Fact**
In children with SDNS, it is essential to minimize the cumulative corticosteroid dose to reduce the risk of long-term complications such as Cushingoid features, hypertension, and cataracts. Alternative immunosuppressive agents like cyclophosphamide should be considered to maintain remission and reduce the reliance on corticosteroids.

**Correct Answer: C. Cyclophosphamide.**