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: He had been prescribed telithromycin 400 mg / d PO to treat an upper respiratory tract infection 7 days prior .
### Output Text:
He had been prescribed <span class="chemical">telithromycin</span> 400 mg / d PO to treat an upper respiratory tract infection 7 days prior . 