A 21-year-old female presents because her urine has turned a brown color. She states that about 2 months ago her urine turned brown 2 days after a cold and stayed brown for about 3 days. At the current time a urinalysis reveals blood with red cells and red cell casts. Fuher laboratory tests include a complete blood count (CBC), serum electrolytes, BUN, creatinine, glucose, antinuclear antibodies (ANAs), and serum complement levels (C3 and C4). All of these tests are within normal limits. Immunofluorescence examination of a renal biopsy from this patient reveals the presence of large, irregular deposits of IgA/C3 in the mesangium. A linear staining pattern is not found. What is the most likely diagnosis for this patient?

**Core Concept**
The patient's presentation of episodic hematuria following a respiratory tract infection, along with laboratory findings of red cell casts and immunofluorescence examination of renal biopsy showing IgA/C3 deposits in the mesangium, suggests an immune-mediated glomerulonephritis.

**Why the Correct Answer is Right**
The patient's symptoms and laboratory findings are consistent with IgA nephropathy (Berger's disease), a common cause of recurrent hematuria. The presence of IgA/C3 deposits in the mesangium on immunofluorescence examination is diagnostic for IgA nephropathy. This condition is characterized by the deposition of IgA antibodies in the glomeruli, leading to inflammation and damage. The linear staining pattern is not found, which helps to rule out Goodpasture syndrome. The normal laboratory tests, including ANAs and serum complement levels, also support the diagnosis of IgA nephropathy.

**Why Each Wrong Option is Incorrect**
**Option A:** Goodpasture syndrome can present with hematuria, but it is typically associated with a linear staining pattern on immunofluorescence examination, which is not found in this patient.
**Option B:** Lupus nephritis is a common cause of glomerulonephritis, but it is typically associated with ANA positivity and low serum complement levels, which are not present in this patient.
**Option C:** Membranous nephropathy is characterized by the deposition of immune complexes on the glomerular basement membrane, which is not consistent with the patient's immunofluorescence findings.

**Clinical Pearl / High-Yield Fact**
IgA nephropathy is the most common cause of recurrent hematuria worldwide, and it is often associated with respiratory tract infections. The condition can progress to chronic kidney disease if left untreated.

**Correct Answer:** C. IgA nephropathy

✓ Correct Answer: A. Berger's disease