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
Which of the following is the most common organism found in retreatment cases of periapical abscess? 
Select and explain. 
A. E. faecalis, B. Trepenoma denticola, C. S. mutans, D. Actinomyces
A: E. faecalis
Answer: A: E. faecalis 
Explanation: PERSISTENT APICAL PERIODONTITIS

It is post-treatment apical periodontitis in an endodontically treated tooth (re-treatment case).
Enterococcus faecalis is found the most consistently reported organism in persistent apical periodontitis.