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: Severe nelarabine neurotoxicity in a patient who received concurrent intrathecal ( IT ) chemotherapy is reported .
### Output Text:
Severe <span class="chemical">nelarabine</span> neurotoxicity in a patient who received concurrent intrathecal ( IT ) chemotherapy is reported . 