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: We sought to determine if prenatal cocaine exposure increases the incidence of subependymal cysts in preterm infants .
### Output Text:
We sought to determine if prenatal <span class="chemical">cocaine</span> exposure increases the incidence of subependymal cysts in preterm infants . 