A 23-year-old male was seen in the emergency department after suffering 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 tomography(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 informed that the patient had a large amount of urine output in the last 24 hours. The nursing records reported his urine output over the previous 24 hours to be 5000 cc. He had not been given any diuretic medications. A urine osmolality was ordered and was found to be low. His physician remarked that the kidneys were not concentrating urine normally.
What is the most likely diagnosis for the increasing dilute urine output?
Which biochemical mediator is responsible for this disorder ?