A 24-year-old woman with type 1 diabetes mellitus presents with abdominal pain, vomiting and lethargy. On examination, she is tachypnoeic with Kussmaul respirations and has a fruity breath odour. Capillary blood glucose is 24 mmol/L and ketones are 5.0 mmol/L. Venous blood gas shows pH 7.12 and bicarbonate 10 mmol/L. According to standard DKA protocols, what is the most appropriate initial fluid resuscitation?