A 68-year-old man presents with progressive lower urinary tract symptoms including nocturia (×4 per night), weak urinary stream, and hesitancy for 18 months. Digital rectal examination reveals a smooth, enlarged, rubbery prostate. Serum PSA is 2.8 ng/mL. Which investigation is most appropriate to confirm the diagnosis of benign prostatic hyperplasia and assess the degree of bladder outlet obstruction?