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: There is still no proof of causative effect of VPA in patients with encephalopathy , but only of an association with an assumed causal relation .
### Output Text:
There is still no proof of causative effect of <span class="chemical">VPA</span> in patients with encephalopathy , but only of an association with an assumed causal relation . 