A 22-year-old man presents to the Emergency Department with a painless ulcer affecting his
glans penis. He admits to unprotected sexual intercourse with a man some two weeks earlier.
There is no other past medical history of note.
Examination reveals a single oval, painless ulcer, surrounded by a bright erythematous
margin. The base is clean and it is discharging clear serum. Dark field microscopy of ulcer
fluid demonstrates spirochaetes. He is penicillin-allergic.

Which is the most appropriate antibiotic choice?