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: Differential diagnosis of high serum creatine kinase levels in systemic lupus erythematosus .
### Output Text:
Differential diagnosis of high serum <span class="chemical">creatine</span> kinase levels in systemic lupus erythematosus . 