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Periapical periodontitis may develop into a periapical abscess , where a collection of pus forms at the end of the root , the consequence of spread of infection from the tooth pulp ( odontogenic infection ) , or into a periapical cyst , where an epithelial lined , fluid filled structure forms .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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