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: Patients taking hydroxychloroquine can demonstrate a toxic reaction in the retina despite the absence of known risk factors .
### Output Text:
Patients taking <span class="chemical">hydroxychloroquine</span> can demonstrate a toxic reaction in the retina despite the absence of known risk factors . 