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: OBJECTIVE : To assess whether prednisone ( PDN ) produces anxiety and / or cerebral glial changes in rats .
### Output Text:
OBJECTIVE : To assess whether <span class="chemical">prednisone</span> ( <span class="chemical">PDN</span> ) produces anxiety and / or cerebral glial changes in rats . 