A 62-year-old woman presents to the emergency department with a headache, slight blurring of vision, and mildly altered sensorium. She states that her symptoms began the day before and had slowly worsened. She says that over the past 4 to 5 days, she’s had multiple episodes of diarrhea.

The patient has type 2 diabetes mellitus, which is controlled on daily metformin, and benign essential hypertension, for which she takes hydrochlorothiazide 12.5 mg daily. On physical exam, her vital signs are stable (BP, 126/82 mm Hg; heart rate, 80 beats/min; respiratory rate, 8 breaths per minute) and she has no focal findings. Laboratory studies include: Na+ 138 mEq/L (135-145 mEq/L); K+ 3.8 mEq/L (3.0-4.5 mEq/L); Cl- 107 mEq/L (97-107 mEq/L); HCO3- 19 mmol/L (20-32 mmol/L); glucose 245 mg/dL (70-110 mg/dL); serum ketones negative; and venous pH 7.26.