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?

Inappropriate secretion of antidiuretic hormone is suggested in a patient without clinical evidence of volume depletion or an edematous (i.e., salt-retaining) condition. This syndrome may be idiopathic, associated with certain pulmonary and intracranial pathologies, resulting from endocrine disorders (e.g., hypothyroidism), or drug-induced (e.g., many psychotropic agents). Volume depletion is unlikely in the absence of orthostatic hypotension. Psychogenic polydipsia requires the ingestion of huge quantities of water to overcome the kidneys' ability to excrete a free-water load and would be associated with a very dilute urine (i.e., urine specific gravity of 1.001 or 1.002). Cirrhosis is unlikely in the absence of ascites and edema. Congestive heart failure can cause hyponatremia but would be associated with edema and evidence of venous congestion.