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: RESULTS : Anxiety was documented in both groups of PDN treated rats compared with controls .
### Output Text:
RESULTS : Anxiety was documented in both groups of <span class="chemical">PDN</span> treated rats compared with controls . 