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: Neonatal pyridoxine responsive convulsions due to isoniazid therapy .
### Output Text:
Neonatal <span class="chemical">pyridoxine</span> responsive convulsions due to <span class="chemical">isoniazid</span> therapy . 