What diagnostic finding is most consistent with a diagnosis of Addison's disease?

Addison's disease, also known as primary adrenal insufficiency, is characterized by insufficient production of adrenal hormones, particularly cortisol and aldosterone. One of the hallmark findings in Addison's disease is an imbalance in electrolytes due to the lack of aldosterone, which normally promotes sodium retention and potassium excretion.

A serum sodium level of 128 mEq/L is significantly low and indicates hyponatremia, which is a common laboratory finding in patients suffering from Addison's disease. The condition typically leads to elevated levels of potassium (hyperkalemia) due to decreased aldosterone, which would shift potassium regulation. Therefore, a very low serum sodium level aligns with the expected presentation in Addison's disease.

While other options may reflect various aspects of adrenal insufficiency, the specific finding of low serum sodium levels strongly correlates with the pathology of Addison's disease, making it the most consistent diagnostic indicator of this condition.