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: We consider this episode of SIADH to be the result of a combination of factors including a weakness of the central nervous system and the long - term administration of VPA .
### Output Text:
We consider this episode of SIADH to be the result of a combination of factors including a weakness of the central nervous system and the long - term administration of <span class="chemical">VPA</span> . 