A 32-year-old male presents to the emergency department 8 hours after a crush injury to the right leg from a fallen concrete slab. He has severe pain, swelling, and tense compartments on clinical examination. Serum creatinine is 1.8 mg/dL (baseline 0.9), potassium 6.2 mEq/L, and urine dipstick shows 3+ blood but RBC count is 0–2/hpf. What is the most appropriate immediate next step in management?