A 23-year-old woman presents with weight loss, polyuria and polydipsia over 4 weeks. Capillary glucose is 25 mmol/L, ketones are 4.0 mmol/L and venous blood gas shows pH 7.18 and bicarbonate 12 mmol/L. She is haemodynamically stable but dehydrated. What is the most appropriate initial intravenous fluid for resuscitation?