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: Within the 48 h following the gradual introduction of quetiapine ( up to 600 mg / day ) , the patient presented severe agitation without an environmental explanation , contrasting with the absence of a history of aggressiveness or personality disorder .
### Output Text:
Within the 48 h following the gradual introduction of <span class="chemical">quetiapine</span> ( up to 600 mg / day ) , the patient presented severe agitation without an environmental explanation , contrasting with the absence of a history of aggressiveness or personality disorder . 