A 65-year-old woman presented to the emergency room complaining of increasing confusion and lethargy over the past week. She was recently diagnosed with small cell cancer of the lung. She has been afebrile and had no other recent illnesses. She is not taking any medications. Her blood pressure is 136/82 mm Hg, heart rate is 84 bpm, and respiratory rate is 14 breaths/minute and unlabored.
On examination, she is an older woman, difficult to arouse, and reacts only to painful stimuli. She can move her extremities without apparent motor deficits, and her deep tendon reflexes are decreased symmetrically. Other examinations are normal, with normal jugular venous pressure and no extremity edema. Laboratory test results reveal: Serum sodium: 108 mmol/L Potassium: 3.8 mmol/L Bicarbonate: 24 mEq/L Blood urea nitrogen: 5 mg/dL Creatinine: 0.5 mg/dL Serum osmolality: 220 mOsm/kg Urine osmolality:…
