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: Introduction of tacrolimus as an alternative immunosuppressive agent resulted in the recurrence of TMA and the subsequent loss of the renal allograft .
### Output Text:
Introduction of <span class="chemical">tacrolimus</span> as an alternative immunosuppressive agent resulted in the recurrence of TMA and the subsequent loss of the renal allograft . 