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: Despite concerns of fatal bone marrow toxicity due to carbamazepine , this is only the fourth documented and published report .
### Output Text:
Despite concerns of fatal bone marrow toxicity due to <span class="chemical">carbamazepine</span> , this is only the fourth documented and published report . 