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: These findings are consistent with the postulated mechanism for this unusual syndrome : acute diffuse crystallization of uric acid in renal tubules .
### Output Text:
These findings are consistent with the postulated mechanism for this unusual syndrome : acute diffuse crystallization of <span class="chemical">uric acid</span> in renal tubules . 