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: Acute vanishing bile duct syndrome is a rare but established cause of progressive cholestasis in adults , is most often drug or toxin related , and is of unknown pathogenesis .
### Output Text:
Acute vanishing bile duct syndrome is a rare but established cause of progressive cholestasis in adults , is most often drug or toxin related , and is of unknown pathogenesis . 