In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Fever and subsequent multiple painful erythematous nodules over extremities developed on D11 , D16 , D17 , and D19 , respectively , after ATRA therapy .
### Output Text:
Fever and subsequent multiple painful erythematous nodules over extremities developed on D11 , D16 , D17 , and D19 , respectively , after <span class="chemical">ATRA</span> therapy . 