Q. A 35-year-old male presents to the emergency department with a chief complaint of increased thirst and frequent urination for the past 2 days. The patient reports that he has been drinking large amounts of water, up to 3 to 4 gallons per day, and urinating frequently, up to 10 to 12 times per day. He denies any nausea, vomiting, abdominal pain, or fever. He also reports that he has been feeling tired and weak. His past medical history is significant for type 1 diabetes, which was diagnosed 10 years ago and has been managed with insulin therapy. He also has a history of hypertension. His current home medications are insulin glargine 30 units at bedtime, insulin aspart 10 units with meals, lisinopril 10 mg orally once daily, and hydrochlorothiazide 25 mg orally once daily. Social history is notable for the patient being a non-smoker and occasionally drinking alcohol. He is currently unemployed and lives alone. On physical examination, the patient is alert and oriented, with stable vital signs. He appears dehydrated with dry mucous membranes. The rest of the examination is unremarkable. You order blood work and a urinalysis. Laboratory results: Serum glucose: 200 mg/dL Sodium: 130 mEq/L Potassium: 3.5 mEq/L Chloride: 90 mEq/L Bicarbonate: 30 mEq/L Urine osmolality: 50 mOsm/kg Urine specific gravity: 1.005 Based on the patient's symptoms and laboratory values, which of the following diagnostic tests is most indicated at this time?