A 32-year-old man is feeling unwell and brought to the hospital. His serum sodium is 125 mEq/L. On examination, his blood pressure is 110/70 mmHg, pulse 110/min supine, and sitting up causes him to feel lightheaded with a drop in his blood pressure. His urine sodium is 5 mEq/L and urine osmolality is 800 mOsm/kg.For the above patient with hyponatremia, select the most likely diagnosis.