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: Thirty - five patients ( 13 . 4 % ) had visual field abnormalities , which were attributed to hydroxychloroquine treatment in 4 patients ( 1 . 5 % ) .
### Output Text:
Thirty - five patients ( 13 . 4 % ) had visual field abnormalities , which were attributed to <span class="chemical">hydroxychloroquine</span> treatment in 4 patients ( 1 . 5 % ) . 