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 magnetic resonance imaging ( MRI ) of her spine demonstrated changes from C2 to C6 consistent with subacute combined degeneration .
### Output Text:
A magnetic resonance imaging ( MRI ) of her spine demonstrated changes from C2 to C6 consistent with subacute combined degeneration . 