**Core Concept**
The patient's presentation of gross hematuria, loin pain, and diarrhea, with normal serum C3 levels and a normal urine examination, suggests a renal condition that may be related to an infectious or inflammatory cause.

**Why the Correct Answer is Right**
The combination of hematuria, loin pain, and diarrhea in a child is suggestive of IgA nephropathy, also known as Berger's disease. IgA nephropathy is a form of glomerulonephritis characterized by the deposition of IgA antibodies in the glomeruli. This condition often presents with episodic hematuria and loin pain, and can be associated with gastrointestinal symptoms like diarrhea. Normal serum C3 levels help to differentiate IgA nephropathy from other forms of glomerulonephritis, such as membranoproliferative glomerulonephritis or post-streptococcal glomerulonephritis, which typically present with low C3 levels.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because IgA nephropathy is the most common cause of recurrent hematuria in children, and the patient's symptoms and normal C3 levels are more consistent with this diagnosis.
**Option B:** This option is incorrect because Henoch-Schönlein purpura (HSP) is a form of vasculitis that can present with hematuria, loin pain, and gastrointestinal symptoms, but it typically presents with low C3 levels and palpable purpura.
**Option C:** This option is incorrect because membranoproliferative glomerulonephritis is a different form of glomerulonephritis that typically presents with low C3 levels and renal insufficiency.
**Option D:** This option is incorrect because post-streptococcal glomerulonephritis typically presents with low C3 levels, hypertension, and renal insufficiency, which are not consistent with this patient's presentation.

**Clinical Pearl / High-Yield Fact**
IgA nephropathy is a common cause of recurrent hematuria in children, and normal serum C3 levels can help to differentiate it from other forms of glomerulonephritis.

**Correct Answer:** C. IgA nephropathy.