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: Although a causal relationship between the use of an anthraquinone - containing herbal agent and renal injury remains to be proven , phytotherapy - associated interstitial nephropathy should be considered in patients who present with unexplained renal failure .
### Output Text:
Although a causal relationship between the use of an <span class="chemical">anthraquinone</span> - containing herbal agent and renal injury remains to be proven , phytotherapy - associated interstitial nephropathy should be considered in patients who present with unexplained renal failure . 