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: It is well known that ceftriaxone leads to pseudolithiasis in some patients .
### Output Text:
It is well known that <span class="chemical">ceftriaxone</span> leads to pseudolithiasis in some patients . 