A 47-year-old man presents to his physician because he has noticed that his right breast is noticeably larger than his left breast. Breast examination demonstrates that the right breast is diffusely enlarged. The breast tissue is freely mobile and without distinct masses. The patient repos that the breast has been slowly enlarging over a period of several years. Which of the following is the MOST likely mechanism causing a relative excess of hormone leading to the breast enlargement in this patient?