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: The patient had suffered a previous episode of " acute hepatitis of unknown origin , " that occurred after telithromycin usage .
### Output Text:
The patient had suffered a previous episode of " acute hepatitis of unknown origin , " that occurred after <span class="chemical">telithromycin</span> usage . 