A 23-year-old sexually active male presents with a white patch with red borders on the oral mucosa that is very sensitive to touch. Which organism is most likely responsible?

Correct answer: D. Treponema pallidum

• A. HPV causes verrucous (wart-like) papillary lesions; it does not produce a painful plaque with defined red borders.
• B. Herpes simplex virus causes vesicles that rupture into ulcers, typically in clusters, not a single white patch with red borders.
• C. Candida albicans causes pseudomembranous candidiasis (white patches that wipe off) or erythematous candidiasis. It does not cause a focal lesion with red borders in a sexually active patient.
• D. CORRECT. The presentation of a painful, red-bordered lesion in a sexually active young male is consistent with a primary syphilitic chancre (hard chancre). Although chancres are classically painless on skin, oral syphilitic lesions can be painful. Treponema pallidum is the causative spirochete.