A 64-year-old man has severe polyuria and polydipsia, drinking 3 to 4 glasses of water and producing over 0.5 liters of urine each hour. Rajesh a new medicine resident places the patient on overnight water restriction for fuher analysis. The test results shown below were obtained the following morning. Plasma sodium concentration: 155 mEq/L Urine osmolarity: 90 mOsmol/L Urine glucose concentration: 0 mg/dL Which of the following is the most likely diagnosis?

Diabetes insipidus is characterized by the excretion of abnormally large volumes of dilute urine (polyuria) with a commensurate increase in fluid intake (polydipsia). The most common type is due to inadequate secretion of antidiuretic hormone (also called vasopressin) and is usually referred to as "neurogenic" diabetes insipidus. This condition rarely causes severe problems as long as the person has plenty of water to drink. Placing the patient on overnight water restriction caused severe dehydration and a greatly elevated plasma sodium concentration. The possibility of diabetes mellitus, which can also be associated with polyuria and polydipsia, is easily excluded by the lack of glucosuria. Addison's disease results from failure of the adrenal coices to produce adrenocoical hormones. The lack of aldosterone leads to decreases in sodium reabsorption allowing large amounts of sodium to be lost into the urine. Polyuria and polydipsia are not characteristic of Addison's disease. Fanconi's syndrome is associated with multiple transpo defects in the proximal tubule. Large amounts of glucose (as well as other substances normally reabsorbed in the proximal tubule) are usually present in the urine.