This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
'Canker sore' is otherwise commonly known as 
Select and explain. 
A. Recurrent apthous ulcer, B. Recurrent herpetic gingivitis, C. Acute necrotizing ulcerative gingivitis, D. Recurrent herpes labialis
A: Recurrent apthous ulcer
Answer: A: Recurrent apthous ulcer 
Explanation: None