A 10-year-old previously healthy child has been noted by her parents to be constantly thirsty. She is consuming large amounts of soft drinks. She is urinating often. Her diet and exercise patterns have not changed, except for an increased appetite, yet she appears cachectic and has lost 7 kg over the past 4 months. On physical examination there are no abnormal findings, other than peripheral muscle wasting and weakness. Which of the following laboratory findings would you most strongly suspect is present in this girl?