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: This case documents acute drug - related vanishing bile duct syndrome in the pediatric age group and suggests shared immune mechanisms in the pathogenesis of both Stevens - Johnson syndrome and vanishing bile duct syndrome .
### Output Text:
This case documents acute drug - related vanishing bile duct syndrome in the pediatric age group and suggests shared immune mechanisms in the pathogenesis of both Stevens - Johnson syndrome and vanishing bile duct syndrome . 