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 week after this second cycle , she noted numbness in her lower extremities .
### Output Text:
A week after this second cycle , she noted numbness in her lower extremities . 