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: In a prevention study , rats received 4 days of LF treatment followed by Dex and continued during the test period .
### Output Text:
In a prevention study , rats received 4 days of LF treatment followed by <span class="chemical">Dex</span> and continued during the test period . 