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: In 2 of the 4 patients , rechallenge with warfarin led to recurrence of the lesions .
### Output Text:
In 2 of the 4 patients , rechallenge with <span class="chemical">warfarin</span> led to recurrence of the lesions . 