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: Cerebral vasculitis following oral methylphenidate intake in an adult : a case report .
### Output Text:
Cerebral vasculitis following oral <span class="chemical">methylphenidate</span> intake in an adult : a case report . 