A 32-year-old male was seen in the emergency department yesterday after suf-
fering a concussion and head trauma from a motor vehicle accident. The
patient was stabilized in the emergency department and transferred to the
intensive care unit (ICU) for observation. The patient had computed tomogra-
phy (CT) scan of the head that revealed a small amount of cerebral edema but
was otherwise normal. During the second day in the ICU, the nurse informs
you that the patient has had a large amount of urine output in the last 24 hours.
The nursing records report his urine output over the last 24 hours was 6400 cc.
He has been given no diuretic medications. A urine osmolality was ordered
and was found to be low. His physician remarks that the kidneys are not con-
centrating urine normally.
What is the most likely diagnosis for the increasing dilute urine
What is the biochemical mediator that is responsible for this