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: We examined the case of a lithium - treated patient who had recurrent hypercalcemia to better understand the disease process .
### Output Text:
We examined the case of a <span class="chemical">lithium</span> - treated patient who had recurrent hypercalcemia to better understand the disease process . 