A 60-year-old woman is brought to the ER by her family for increasing confusion and lethargy over the past week. Recently, she was diagnosed with small cell cancer of the lung. BP: 136/82 mm Hg HR: 84 bpm RR: 14 breaths per minute She is afebrile. On further examination, she looks an elderly appearing woman who is difficult to arouse and reacts only to painful stimuli. She is able to move her extremities without apparent motor deficits, and her deep tendon reflexes are decreased symmetrically. She has a normal jugular venous pressure and no extremity edema. Laboratory results : Serum sodium level: 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 is 220 mOsm/kg, and urine osmolality is 400 mOsm/kg. A computed tomographic (CT) scan of the brain shows no masses or hydrocephalus. What is the likely diagnosis?…
