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: A 77 - year - old woman with refractory multiple myeloma was treated with a 4 - day continuous intravenous infusion of vincristine and doxorubicin and 4 days of oral dexamethasone .
### Output Text:
A 77 - year - old woman with refractory multiple myeloma was treated with a 4 - day continuous intravenous infusion of <span class="chemical">vincristine</span> and <span class="chemical">doxorubicin</span> and 4 days of oral <span class="chemical">dexamethasone</span> . 