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 pathogenesis of 5 - fluorouracil neurotoxicity may be due to a Krebs cycle blockade by fluoroacetate and fluorocitrate , thiamine deficiency , or dihydrouracil dehydrogenase deficiency .
### Output Text:
The pathogenesis of <span class="chemical">5 - fluorouracil</span> neurotoxicity may be due to a Krebs cycle blockade by <span class="chemical">fluoroacetate</span> and <span class="chemical">fluorocitrate</span> , <span class="chemical">thiamine</span> deficiency , or <span class="chemical">dihydrouracil</span> dehydrogenase deficiency . 