A 45-yeаr-оld mаle presents with cоmplаints оf unexplained weight gain, fatigue, and increased thirst. Upon physical examination, he shows signs of mild edema and elevated blood pressure. Laboratory tests reveal a low plasma sodium level, elevated potassium level, and high levels of plasma cortisol. Further testing indicates the presence of adrenal insufficiency. Based on this case, which adrenal hormone is most likely involved in this patient's symptoms, and what is its primary function?