19 y/o Asian man has Hematuria. He has always been healthy with no medical problems; but yesterday, he developed an upper respiratory infection. Last night, when he urinated, he noted that his urine was initially very dark and then appear to be grossly bloody. This is the first time this has ever happened, and he has no family hx of any similar problems. His physical examination is entirely normal. CBC and electrolytes are normal. His serum creatinine is 0.8mg/dl. Urinalysis demonstrated blood-tinged urine with 4+ blood and trace protein. There are too many red blood cells to count microscopically and there are occasional red blood cell casts. What is the most likely cause of his hematuria?