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: Nelarabine was felt to be the cause of her symptoms .
### Output Text:
<span class="chemical">Nelarabine</span> was felt to be the cause of her symptoms . 