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: We present the case of a 28 - year - old man on chronic warfarin therapy who sustained a minor muscle tear and developed increasing pain and a flexure contracture of the right hip .
### Output Text:
We present the case of a 28 - year - old man on chronic <span class="chemical">warfarin</span> therapy who sustained a minor muscle tear and developed increasing pain and a flexure contracture of the right hip . 