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: Dental
Inflammation of the periapical tissue is sustained by: 
Select and explain. 
A. Stagnant tissue fluid, B. Necrotic tissue, C. Microorganisms, D. Pus cells
C: Microorganisms
Answer: C: Microorganisms 
Explanation: Apical periodontitis is a chronic inflammatory disorder of periradicular tissues caused by aetiological agents of endodontic origin. 
Persistent apical periodontitis occurs when root canal treatment of apical periodontitis has not adequately eliminated the intraradicular infection (microbes).