Okay, let's tackle this question. The patient is a six-year-old male with recurrent gross hematuria for two years. No history of burning micturition or pyuria, and urine exam shows no pus cells. Culture is sterile, and serum C3 is normal.

First, I need to think about the possible causes of hematuria in children. Since there's no infection (sterile culture, no pus cells), infectious causes like UTIs are out. The absence of C3 deficiency is important. C3 is part of the complement system, so low C3 is seen in conditions like post-streptococcal glomerulonephritis or membranoproliferative glomerulonephritis (MPGN). But here, C3 is normal, so those might not be the case.

Recurrent hematuria without other symptoms like proteinuria or hypertension might point to a genetic condition. Alport syndrome comes to mind. It's an X-linked disorder causing hematuria, often presenting in childhood. It's associated with sensorineural hearing loss and eye abnormalities. Since the patient is male, X-linked recessive would fit. But the question doesn't mention other symptoms, but maybe they aren't present yet.

Another possibility is thin basement membrane disease, which is a benign condition causing asymptomatic hematuria. It's common in children and often familial. The hematuria is usually persistent but not associated with other symptoms. No C3 deficiency here.

IgA nephropathy can present with hematuria, often after upper respiratory infections. However, it's more common in older children and adolescents. The presence of recurrent episodes might fit, but the absence of C3 is a clue. In IgA, C3 is usually normal, but there might be other findings like proteinuria or hypertension which aren't mentioned here.

Other possibilities: Sickle cell disease can cause hematuria, but that's more common in certain ethnic groups. Also, not typically recurrent unless there's other pathology. Hereditary nephritis (Alport) is a strong possibility. The key here is the recurrent nature and normal C3. Alport is associated with normal C3 levels, while MPGN or post-strep GN would have low C3. Since the question says serum C3 is normal, Alport is more likely. Thin basement membrane is another option, but it's usually less severe and might not present with gross hematuria as often.

So the most probable diagnosis is Alport syndrome. The correct answer would be the option corresponding to Alport. The other options (like IgA nephropathy, thin basement membrane, or MPGN) can be ruled out based on the given information.

**Core Concept**
Recurrent gross hematuria in a child without infection, pyuria, or low C3 levels suggests a **hereditary glomerular basement membrane disorder** like Alport syndrome. This condition is characterized by **X-linked recessive inheritance**, hematuria, and progressive renal failure, often associated with **sensorineural hearing loss** and **eye abnormalities**.

**Why the Correct Answer is Right**
**Alport syndrome** is caused by mutations in the *COL4A5* gene, leading to defective type IV collagen in glomerular basement membranes. The hematuria is due to **structural fragility** of the glomeruli, allowing red blood

✓ Correct Answer: B. IgA nephropathy