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: Thymus weight was used as a marker of glucocorticoid activity , and serum urate to assess XO inhibition .
### Output Text:
Thymus weight was used as a marker of glucocorticoid activity , and serum <span class="chemical">urate</span> to assess XO inhibition . 