A 28-year-old man comes to the clinic for decreased urine output and malaise. One week ago, he started a new medication for sinusitis. He now has low-grade fever and a diffuse, pruritic, erythematous maculopapular rash. Physical examination is notable for mild bilateral costovertebral angle tenderness. Laboratory studies show BUN 48 mg/dL and creatinine 2.6 mg/dL (baseline creatinine 0.9 mg/dL). Urinalysis shows specific gravity 1.010, 2+ blood, 1+ protein, and numerous WBCs; urine culture shows no growth. Which of the following additional findings would be most characteristic of the underlying renal pathology?

(Assume no urinary tract obstruction and no history of chronic kidney disease.)