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: The most frequently observed side effect was fecal occult blood ( 25 % of patients ) ; however , there was no other evidence of gastrointestinal ( GI ) bleeding in these patients .
### Output Text:
The most frequently observed side effect was fecal occult blood ( 25 % of patients ) ; however , there was no other evidence of gastrointestinal ( GI ) bleeding in these patients . 