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: These brain substrates may help account for the symptoms of MA abuse , providing therapeutic targets for drug - induced brain injury .
### Output Text:
These brain substrates may help account for the symptoms of <span class="chemical">MA</span> abuse , providing therapeutic targets for drug - induced brain injury . 