A 16-year-old girl with known Type 1 Diabetes Mellitus presents to the ED feeling unwell with nausea, vomiting, and abdominal pain. Her capillary blood glucose is 25 mmol/L and ketones are 3+ on urine dipstick. Her pH on venous blood gas is 7.20 and bicarbonate is 12 mmol/L. What is the most critical initial management fluid according to national guidelines?