A 78-year-old woman is admitted with confusion and drowsiness. Her observations are stable. Blood tests show: Na$^+$ 118 mmol/L, K$^+$ 3.9 mmol/L, Urea 4.2 mmol/L, Creatinine 75 $\mu$mol/L, Serum osmolality 255 mOsmol/kg. Urine osmolality is 450 mOsmol/kg and urine sodium is 60 mmol/L. She was recently started on sertraline for depression. What is the most likely diagnosis?