A 58-year-old man with diabetes mellitus presents to the emergency department 6 hours after accidental ingestion of ethylene glycol. He is confused, has a respiratory rate of 28/min, and blood pH is 7.18. Serum creatinine is 2.8 mg/dL (baseline 0.9 mg/dL). Urinalysis shows muddy brown casts and granular casts. What is the most appropriate immediate next step in management?