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 studied the role of xanthine oxidase ( XO ) , which is implicated in the production of reactive oxygen species , in dexamethasone - induced hypertension ( dex - HT ) .
### Output Text:
We studied the role of <span class="chemical">xanthine</span> oxidase ( XO ) , which is implicated in the production of reactive oxygen species , in <span class="chemical">dexamethasone</span> - induced hypertension ( <span class="chemical">dex</span> - HT ) . 