A mother brings her 5-year-old boy to the emergency department because of general malaise and a swelling on his scrotum that was not present yesterday. The mother says the child is not himself, and he had a low-grade fever and cough 2 weeks ago. There is no significant past medical history. On examination, the child is ill-looking and lethargic. He has palpable purpura on the buttocks and the lower legs. There is swelling of the right scrotum that is slightly tender to palpation. An abdominal examination reveals moderate tenderness to palpation. His urine contains a mild amount of red blood cells and protein. Which potential complication necessitates admission?