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: A newborn with massive tricuspid regurgitation , atrial flutter , congestive heart failure , and a high serum lithium level is described .
### Output Text:
A newborn with massive tricuspid regurgitation , atrial flutter , congestive heart failure , and a high serum <span class="chemical">lithium</span> level is described . 