MRCP2-0935

A 32-year-old man presents to the clinic complaining of painful and itchy ulcers on his penis for the past 3 days. He also experienced flu-like symptoms in the previous week, dysuria, and a white discharge from his urethra. He had unprotected anal sex with a casual partner 12 days ago.

During the examination, the doctor observed three tender, shallow ulcers under the foreskin, along with redness of the surrounding skin. The urine dipstick test was normal. The urethral swab microscopy showed the presence of pus cells.

Which test is most likely to provide a definitive diagnosis?