A 26-year-old man presents to his primary care physician complaining of impotence. He reports that he has a healthy, long-term relationship with a woman whom he hopes to marry, but he is embarrassed that he is unable to have an erection. Which of the following is the next best step?
Evaluate nocturnal tumescence
Duplex penile ultrasound
Prescribe sildenafil
Prescribe vardenafil