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: A 61 year old female developed fatal hepatic failure after phenytoin administration .
### Output Text:
A 61 year old female developed fatal hepatic failure after <span class="chemical">phenytoin</span> administration . 