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: Comparison of the patients with or without pseudolithiasis revealed no significant difference with respect to age , sex , duration of the treatment and starvation variables .
### Output Text:
Comparison of the patients with or without pseudolithiasis revealed no significant difference with respect to age , sex , duration of the treatment and starvation variables . 