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: In this study , we prospectively evaluated the incidence and clinical importance of pseudolithiasis in paediatric surgical patients receiving ceftriaxone treatment , who often had to fast in the post - operative period .
### Output Text:
In this study , we prospectively evaluated the incidence and clinical importance of pseudolithiasis in paediatric surgical patients receiving <span class="chemical">ceftriaxone</span> treatment , who often had to fast in the post - operative period . 