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: To our knowledge , this is the first case of acitretin - induced dysphonia .
### Output Text:
To our knowledge , this is the first case of <span class="chemical">acitretin</span> - induced dysphonia . 