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: She had developed a severe acute major depression disorder almost immediately thereafter , possibly related to the use of a serotonin antagonist .
### Output Text:
She had developed a severe acute major depression disorder almost immediately thereafter , possibly related to the use of a <span class="chemical">serotonin</span> antagonist . 