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 17 - day - old infant on isoniazid therapy 13 mg / kg daily from birth because of maternal tuberculosis was admitted after 4 days of clonic fits .
### Output Text:
A 17 - day - old infant on <span class="chemical">isoniazid</span> therapy 13 mg / kg daily from birth because of maternal tuberculosis was admitted after 4 days of clonic fits . 