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: Possible pathophysiological mechanisms which may have been operative in this case include : a direct central nervous system ( CNS ) toxic effect of valproic acid ; a paradoxical epileptogenic effect secondary to the drug ; and an indirect CNS toxic effect mediated through valproic acid - induced hyperammonemia .
### Output Text:
Possible pathophysiological mechanisms which may have been operative in this case include : a direct central nervous system ( CNS ) toxic effect of <span class="chemical">valproic acid</span> ; a paradoxical epileptogenic effect secondary to the drug ; and an indirect CNS toxic effect mediated through <span class="chemical">valproic acid</span> - induced hyperammonemia . 