This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Oral cavity, Subject name: Surgery
A patient has small, oval multiple ulcers in oral cavity with red erythematous margins. The diagnosis is: 
Select and explain. 
A. Carcinoma, B. Tubercular ulcer, C. Aphthous ulcer, D. Syphilitic ulcer
C: Aphthous ulcer
Answer: C: Aphthous ulcer 
Explanation: Aphthous ulcer Small and oval multiple ulcers in oral cavity Has red erythematous margins Syphilitic ulcer Firm, painless, non-itchy skin ulceration Tubercular ulcer Firm, nontender erythematous nodules that soften, ulcerate, and form sinuses.