**Core Concept**
IgA nephropathy, also known as Berger's disease, is a kidney disease characterized by the deposition of IgA antibodies in the glomeruli, leading to inflammation and damage. The condition often presents with recurrent episodes of gross hematuria, particularly following upper respiratory tract infections.

**Why the Correct Answer is Right**
The management of IgA nephropathy involves a multidisciplinary approach, including close monitoring of renal function and proteinuria. Given the low proteinuria level of 130 mg/day, the patient is not currently in the nephrotic range. The correct answer is to initiate corticosteroid therapy, which is the mainstay of treatment for IgA nephropathy, especially in children. Corticosteroids help to reduce inflammation and slow disease progression.

**Why Each Wrong Option is Incorrect**
**Option A:** Angiotensin-converting enzyme (ACE) inhibitors are not the primary treatment for IgA nephropathy, although they may be used to manage hypertension and proteinuria in some cases.
**Option B:** Plasmapheresis is a treatment for severe cases of IgA nephropathy, such as rapidly progressive glomerulonephritis, but it is not the most appropriate next step in this scenario.
**Option C:** Immunosuppressive agents, such as cyclophosphamide, may be used in severe cases of IgA nephropathy, but they are not the first line of treatment for a child with low proteinuria levels.

**Clinical Pearl / High-Yield Fact**
In children with IgA nephropathy, corticosteroid therapy can improve renal outcomes and reduce the risk of progression to end-stage renal disease. Close monitoring of renal function and proteinuria is essential to adjust treatment accordingly.

**Correct Answer:** C. Corticosteroid therapy is the most appropriate next step in the management of this patient with IgA nephropathy and low proteinuria levels.