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: Patients underwent regular measurement of prostate - specific antigen ( PSA ) , urea and electrolytes , serum bFGF and VEGF .
### Output Text:
Patients underwent regular measurement of prostate - specific antigen ( PSA ) , <span class="chemical">urea</span> and electrolytes , serum bFGF and VEGF . 