A 42-year-old woman with diabetic ketoacidosis (pH 7.15, HCO₃⁻ 10 mEq/L, anion gap 18, glucose 520 mg/dL, serum K⁺ 5.2 mEq/L) is admitted to the ICU. After initiation of IV fluids and insulin infusion, what is the drug of choice for acute correction of severe metabolic acidosis?