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: INTERPRETATION : Tranexamic acid retains its convulsive action within FS .
### Output Text:
INTERPRETATION : <span class="chemical">Tranexamic acid</span> retains its convulsive action within FS . 