A 43-year-old man reports a 3-month history of polyuria, nocturia, and polydipsia. He fell off a ladder and hit his head just prior to the onset of these symptoms. The laboratory studies include the following: Na+=155 mEq/L; K+=3.9 mEq/L; Cl-=111 mEq/L; total carbon dioxide=27 mEq/L; glucose=84 mg/dL; Cr=1.0 mg/dL; and osmolality =350 mOsm/kg. The urine specific gravity is 1.002. The patient is most likely to be deficient in which of the following hormones?