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: Moreover , the addition of dexamethasone to the last two doses of mesna was more efficient than three doses of mesna alone when evaluated microscopically .
### Output Text:
Moreover , the addition of <span class="chemical">dexamethasone</span> to the last two doses of <span class="chemical">mesna</span> was more efficient than three doses of <span class="chemical">mesna</span> alone when evaluated microscopically . 