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: All patients underwent a rapid rule - in protocol that included serial sampling of creatine kinase ( CK ) , CK - MB , and cardiac troponin I ( cTnI ) over eight hours .
### Output Text:
All patients underwent a rapid rule - in protocol that included serial sampling of <span class="chemical">creatine</span> kinase ( CK ) , CK - MB , and cardiac troponin I ( cTnI ) over eight hours . 