**Core Concept**
The underlying condition in this clinical scenario is a renal disorder characterized by recurrent gross hematuria, loin pain, and gastrointestinal symptoms, despite normal serum C3 levels. This presentation suggests an inflammatory process involving the kidneys and the gastrointestinal tract.

**Why the Correct Answer is Right**
The diagnosis is likely IgA Nephropathy (Berger's disease), a common cause of recurrent gross hematuria in children and young adults. IgA Nephropathy is characterized by the deposition of IgA antibodies in the glomeruli, leading to inflammation and damage. The normal serum C3 levels are consistent with this diagnosis, as IgA Nephropathy does not typically involve complement activation. The gastrointestinal symptoms, such as diarrhea, may be due to the presence of IgA antibodies in the gut, leading to inflammation and mucosal damage.

**Why Each Wrong Option is Incorrect**
* **Option B:** This option is incorrect because Henoch-Schönlein Purpura (HSP) typically presents with palpable purpura, abdominal pain, and arthritis, but it often involves low serum C3 levels due to complement activation.
* **Option C:** This option is incorrect because Alport syndrome is a genetic disorder characterized by progressive renal failure, hearing loss, and eye abnormalities, but it does not typically present with gross hematuria and gastrointestinal symptoms.
* **Option D:** This option is incorrect because Minimal Change Disease is a common cause of nephrotic syndrome in children, but it typically presents with massive proteinuria, hypoalbuminemia, and edema, rather than gross hematuria and gastrointestinal symptoms.

**Clinical Pearl / High-Yield Fact**
IgA Nephropathy is the most common cause of recurrent gross hematuria in children and young adults, and it often presents with gastrointestinal symptoms due to the presence of IgA antibodies in the gut. Remember to consider IgA Nephropathy in the differential diagnosis of a patient with recurrent gross hematuria and gastrointestinal symptoms, even if serum C3 levels are normal.

**Correct Answer:** A. IgA Nephropathy