A 28-year-old male marathon runner presents to the emergency department with severe muscle pain, dark urine, and a serum lactate of 8.2 mmol/L (normal: 0.5–2.2 mmol/L) after completing a 42 km race. His liver function tests are normal, and blood glucose is 95 mg/dL. Arterial blood gas shows pH 7.28 with HCO₃⁻ of 14 mEq/L. What is the most appropriate immediate next step in management?