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: Laboratory variables of renal function [ serum creatinine , blood urea nitrogen ( BUN ) and electrolytes ] were evaluated before initiation of therapy and every 2 days , until termination of the study ( a period of at least 6 days ) .
### Output Text:
Laboratory variables of renal function [ serum <span class="chemical">creatinine</span> , <span class="chemical">blood urea nitrogen</span> ( <span class="chemical">BUN</span> ) and electrolytes ] were evaluated before initiation of therapy and every 2 days , until termination of the study ( a period of at least 6 days ) . 