Review Question - QID 109400

A 34-year-old man presents to his primary care physician with frequent urination. He was recently hospitalized following a severe motorcycle accident in which he suffered multiple injuries to his head and extremities. He reports that he has been constantly thirsty and has been urinating four to five times per night since being discharged from the hospital one week prior to presentation. His past medical history is notable for type II diabetes mellitus, which is well controlled on metformin. He has a 10 pack-year smoking history and drinks 3-4 alcoholic beverages per day. His temperature is 98.8°F (37.1°C), blood pressure is 110/70 mmHg, pulse is 95/min, and respirations are 18/min. Physical examination reveals delayed capillary refill and decreased skin turgor. Notable laboratory results are shown below:

Serum:
Na+: 148 mEq/L
Cl-: 101 mEq/L
K+: 3.7 mEq/L
HCO3-: 25 mEq/L
BUN: 20 mg/dL
Glucose: 110 mg/dL
Hemoglobin A1c: 5.7%

This patient’s condition is most likely caused by defective production in which of the following locations?