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: Pretreatment with S . virgaurea extract for 5 weeks at a dose of 250 mg / kg followed by isoproterenol injection significantly prevented the observed alterations .
### Output Text:
Pretreatment with S . virgaurea extract for 5 weeks at a dose of 250 mg / kg followed by <span class="chemical">isoproterenol</span> injection significantly prevented the observed alterations . 