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: Major azotemia ( serum creatinine greater than 2 . 5 mg / dl or BUN greater than 50 mg / dl ) occurred in only 2 % .
### Output Text:
Major azotemia ( serum <span class="chemical">creatinine</span> greater than 2 . 5 mg / dl or BUN greater than 50 mg / dl ) occurred in only 2 % . 