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: After substituting VPA with zonisamide , the serum sodium level returned to normal .
### Output Text:
After substituting <span class="chemical">VPA</span> with <span class="chemical">zonisamide</span> , the serum <span class="chemical">sodium</span> level returned to normal . 