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: Studies of human subjects who have used MA chronically have revealed deficits in dopaminergic and serotonergic systems and cerebral metabolic abnormalities .
### Output Text:
Studies of human subjects who have used <span class="chemical">MA</span> chronically have revealed deficits in dopaminergic and serotonergic systems and cerebral metabolic abnormalities . 