A 23-year-old woman with type 1 diabetes presents with nausea, abdominal pain and polyuria. Capillary glucose is 25 mmol/L. Arterial blood gas shows pH 7.18, bicarbonate 10 mmol/L and anion gap 24 mmol/L. Serum ketones are elevated. What is the most appropriate initial intravenous fluid for resuscitation?