A patient is in recovery after having a benign tumor removed…

A patient is in recovery after having a benign tumor removed from the cerebrum. The patient has had rapid weight gain over three days and his urine output has greatly decreased despite increased fluid intake. The specific gravity (concentration) of his urine is 1.032 (1.005 – 1.030). An electrolyte and hormone panel are ordered. What is most likely the cause of these manifestations?