## **Core Concept**
The question presents a case of a 10-year-old boy with acute onset gross hematuria, loin pain, and diarrhea. The key laboratory findings include normal peripheral smear, normal serum C3 levels, and no decreased urine output. This clinical picture suggests a condition affecting the kidneys, possibly an infection or inflammation.

## **Why the Correct Answer is Right**
The correct answer, **C. Post-streptococcal glomerulonephritis (PSGN) is less likely due to normal C3 levels**, but given the symptoms and lab findings, **the most likely diagnosis is IgA Nephropathy (Berger's disease)**. IgA Nephropathy often presents with gross hematuria following a gastrointestinal infection (like diarrhea), which fits this patient's presentation. The normal serum C3 levels help to rule out conditions like PSGN, which often shows low C3 levels during the acute phase.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Acute tubular necrosis (ATN) typically presents with acute kidney injury (decreased urine output, rising creatinine) and is not primarily associated with gross hematuria or diarrhea as initial symptoms.
- **Option B:** PSGN usually has low C3 levels during the acute phase, which does not match this patient's normal C3 levels.
- **Option D:** Henoch-Schönlein purpura (HSP) can present with renal involvement (hematuria) but is also characterized by purpura, arthritis, and abdominal pain, which are not mentioned here.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **IgA Nephropathy (Berger's disease) is a common cause of recurrent gross hematuria in young adults and children**, often precipitated by infections, particularly those of the upper respiratory or gastrointestinal tract.

## **Correct Answer:** C. IgA Nephropathy (Berger's disease).