Abstract: Difficult and complicated diseases refer to a class of diseases with complex etiology, difficult to diagnose, and difficult to treat. Oral mucosal disease is described as a series of disorders or conditions affecting the mucosa and soft tissue in the oral cavity. In particular, the intricacy of difficult and complicated oral mucosal diseases makes differential diagnosis and diagnosis rather challenging. This paper analyzes the characteristics of difficult and complicated oral mucosal diseases by combined with clinical case studies, and summarize "four must-dos" and "five-direction" diagnostic reasoning process, so as to provide assistance for oral clinicians.

Key words: oral mucosal diseases, complicated and difficult cases, diagnosis, differential diagnosis