A 19-year-old university student presents to the emergency department with polyuria, polydipsia, abdominal pain and vomiting. Vitals: BP 100/65 mmHg, HR 120/min, RR 26/min, T 37.2 °C. Capillary glucose is 25 mmol/L, venous pH 7.12, serum bicarbonate 10 mmol/L and ketones are strongly positive. What is the MOST appropriate initial fluid resuscitation?