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: Forty minutes after receiving a single starting dose of trazodone , a patient developed complete heart block .
### Output Text:
Forty minutes after receiving a single starting dose of <span class="chemical">trazodone</span> , a patient developed complete heart block . 