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: Treatment has included use of plasma exchange , prednisone , aspirin , and dipyridamole .
### Output Text:
Treatment has included use of plasma exchange , <span class="chemical">prednisone</span> , <span class="chemical">aspirin</span> , and <span class="chemical">dipyridamole</span> . 