Aphthous ulcers occur randomly inside the mouth, whereas a fixed drug eruption is defined by its recurrence in the exact same location upon re-exposure to a specific drug.
Behcet's involves recurrent oral and genital ulcers, but they are not fixed to a single location and are part of a systemic vasculitis.
This is an infectious genital ulcer, not a recurrent, drug-induced lesion.
This is a chronic, progressive, "beefy-red" bacterial ulcer, a completely different morphology and etiology than a recurrent FDE plaque.
A syphilitic chancre is a primary infectious ulcer, not a recurrent, drug-induced lesion.
