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: It appears that methylphenidate precipitated the patient ' s pathological behavior .
### Output Text:
It appears that <span class="chemical">methylphenidate</span> precipitated the patient ' s pathological behavior . 