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: Although the mechanisms of peripheral neuropathy are still unclear , high - dose cytosine arabinoside is a therapy that is potentially toxic to the peripheral nervous system , and auto / alloimmunity may play an important role in these mechanisms .
### Output Text:
Although the mechanisms of peripheral neuropathy are still unclear , high - dose <span class="chemical">cytosine arabinoside</span> is a therapy that is potentially toxic to the peripheral nervous system , and auto / alloimmunity may play an important role in these mechanisms . 