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: Mechanisms for protective effects of free radical scavengers on gentamicin - mediated nephropathy in rats .
### Output Text:
Mechanisms for protective effects of free radical scavengers on <span class="chemical">gentamicin</span> - mediated nephropathy in rats . 