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: We report the case of a woman complaining of dysphonia while she was treated by acitretin .
### Output Text:
We report the case of a woman complaining of dysphonia while she was treated by <span class="chemical">acitretin</span> . 