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: Nephrotic syndrome is often accompanied by sodium retention and generalized edema .
### Output Text:
Nephrotic syndrome is often accompanied by <span class="chemical">sodium</span> retention and generalized edema . 