A 71-year-old male farmer presents because he is experiencing a gradual increase of frequency of urination over the previous 4-5 months. He has difficulties starting to urinate, the stream is slow, and he frequently has a sensation of incomplete emptying. He has not had a physical examination in several years. He has no history of sexually transmitted diseases (STDs) or urinary tract infections. Upon examination, his abdomen and his prostate are non-tender. His prostate seems moderately enlarged (estimated at 35 g) but smooth and symmetrical. What diagnostic test do you recommend as the next step in this patient's management?