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: Because of the need for the development of new treatments for Crohn ' s disease , a pilot study was undertaken to estimate the pharmacodynamics and tolerability of fusidic acid treatment in chronic active , therapy - resistant patients .
### Output Text:
Because of the need for the development of new treatments for Crohn ' s disease , a pilot study was undertaken to estimate the pharmacodynamics and tolerability of <span class="chemical">fusidic acid</span> treatment in chronic active , therapy - resistant patients . 