A 15-year-old male with a known history of salt-losing congenital adrenal hyperplasia has been poorly compliant with his glucocorticoid and mineralocorticoid replacement therapy. He presents for his clinic visit with bilateral, firm testicular enlargement. His serum testosterone level is appropriate for his pubertal stage, but his adrenocorticotropic hormone (ACTH) level is markedly elevated. What is the most likely etiology of his testicular masses?