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 case of a nonepileptic patient who developed psychosis following phenytoin treatment for trigeminal neuralgia is described .
### Output Text:
The case of a nonepileptic patient who developed psychosis following <span class="chemical">phenytoin</span> treatment for trigeminal neuralgia is described . 