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 2+ 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?