MS-1023

A 33-year-old man arrives at the emergency department with symptoms of increased thirst and frequent urination. He had suffered a head injury a few days ago and had previously been discharged after investigations. Upon examination, he appears dehydrated and is admitted to a medical ward. The urine osmolality test results show a low level of 250 mosmol/kg after water deprivation and a high level of 655 mosmol/kg after desmopressin administration. Based on this information, where is the deficient substance typically active?