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: A single dose of valproic acid ( VPA ) , which is a widely used antiepileptic drug , is associated with oxidative stress in rats , as recently demonstrated by elevated levels of 15 - F ( 2t ) - isoprostane ( 15 - F ( 2t ) - IsoP ) .
### Output Text:
A single dose of <span class="chemical">valproic acid</span> ( <span class="chemical">VPA</span> ) , which is a widely used antiepileptic drug , is associated with oxidative stress in rats , as recently demonstrated by elevated levels of <span class="chemical">15 - F ( 2t ) - isoprostane</span> ( <span class="chemical">15 - F ( 2t ) - IsoP</span> ) . 