An 8-year-old boy presents with headaches, dizziness, and malaise. He was seen for a severe sore throat 2 weeks ago. Physical examination reveals facial edema. The blood pressure is 180/110 mm Hg. A 24-hour urine collection demonstrates oliguria, and urinalysis shows hematuria. What finding on microscopic urinalysis indicates that hematuria in the patient is caused by a renal process, rather than bleeding from another site in the urinary tract?