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: In a patient receiving phenytoin who presents a viral - like illness , early recognition and discontinuation of the drug are mandatory .
### Output Text:
In a patient receiving <span class="chemical">phenytoin</span> who presents a viral - like illness , early recognition and discontinuation of the drug are mandatory . 