A 29-year-old man with HIV is evaluated in a clinic for a new painful penile ulcer. On examination, there is a 1 cm ulcer on the dorsal shaft of the penis, with irregular borders and a grey exudate on the base. He also has several painful right inguinal lymph nodes. Results of testing for syphilis serology, dark-field microscopy examination, and herpes simplex virus PCR are all negative. At a follow-up visit 1 week later, you send a sample taken from the ulcer for Haemophilus ducreyi culture and meanwhile decide to empirically treat for chancroid.