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: PURPOSE : Prenatal cocaine exposure has been linked with subependymal hemorrhage and the formation of cysts that are detectable on cranial sonography in neonates born at term .
### Output Text:
PURPOSE : Prenatal <span class="chemical">cocaine</span> exposure has been linked with subependymal hemorrhage and the formation of cysts that are detectable on cranial sonography in neonates born at term . 