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: The underlying mechanism of withdrawal - emergent RS in the present case may have been related to the pharmacological profile of risperidone , a serotonin - dopamine antagonist , suggesting the pathophysiologic influence of the serotonin system in the development of RS .
### Output Text:
The underlying mechanism of withdrawal - emergent RS in the present case may have been related to the pharmacological profile of <span class="chemical">risperidone</span> , a <span class="chemical">serotonin</span> - <span class="chemical">dopamine</span> antagonist , suggesting the pathophysiologic influence of the <span class="chemical">serotonin</span> system in the development of RS . 