Final answer:
Chancroid is a sexually transmitted infection that can be mistaken for syphilis due to its ulcer formation on the genitalia.
Step-by-step explanation:
The sexually transmitted infection that may be confused with syphilis due to its ulcer formation on the genitalia is Chancroid. Chancroid is caused by the gram-negative rod Haemophilus ducreyi and is characterized by the development of soft chancres on the genitals or other areas associated with sexual contact.