A 38-year-old male with chronic schizophrenia on a long-term inpatient unit is found sitting on the floor of his room appearing confused and lethargic. Nursing staff report he has been drinking water excessively throughout the day, estimating intake exceeding 10 liters. His weight has increased 4 kg since morning weigh-in. Vital signs include blood pressure 148/92 mmHg, heart rate 78 bpm, and temperature 36.8°C. Laboratory results reveal serum sodium of 118 mEq/L, serum osmolality of 248 mOsm/kg, and urine osmolality of 85 mOsm/kg. He has no history of diabetes insipidus, renal disease, or diuretic use. His current medications include clozapine 400 mg daily and benztropine 1 mg twice daily. Which assessment finding most accurately identifies the mechanism driving this patient's hyponatremia?