A 52-year-old man with diabetes presents to the emergency department following a crush injury to his right leg sustained in a motor vehicle accident. On examination, he is hypotensive and oliguric. Urinalysis shows the finding marked **A** in the diagram — muddy brown granular casts. His serum creatinine has risen from 1.0 to 3.2 mg/dL over 24 hours. Fractional excretion of sodium (FENa) is 4.2%. Which of the following best explains the pathophysiology of the urinary finding marked **A**?