An 80-year-old man comes to the physician for a 4-month history of fatigue. He has lost 10 pounds during this time. Blood pressure is 100/65. Physical examination shows darkening of the skin of the forearms. Laboratory studies show: Na+ 130 mEq/L, K+ 5.5 mEq/L, HCO3– 20 mEq/L, eosinophils 20%. Dysfunction of which of the following layers best explains this patient’s hyponatremia?
A
B
C
D
E
