A 63-year-old man alcoholic with a 50-pack-year history of smoking presents to the emergency room with fatigue and confusion. Physical examination reveals a blood pressure of 110/70 with no orthostatic change. Heart, lung, and abdominal examinations are normal and there is no pedal edema. Laboratory data are as follows:Na: 110 mEq/LK: 3.7 mEq/LCl: 82 mEq/LHCO3: 20 mEq/LGlucose: 100mg/dLBUN: 5 mg/dLCreatinine: 0.7 mg/dLUrinalysis: normalUrine specific gravity: 1.016Which of the following is the most likely diagnosis?