A 48-year-old woman presents with a 2-year history of refractory hypertension and episodes of muscle weakness. Laboratory studies reveal hypokalemia, metabolic alkalosis, suppressed plasma renin activity, and an elevated plasma aldosterone concentration. A CT scan of the abdomen shows bilateral adrenal cortical hyperplasia. Which structure within the adrenal gland is most likely hyperplastic and responsible for her biochemical abnormalities?