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: MDMA polydrug users show process - specific central executive impairments coupled with impaired social and emotional judgement processes .
### Output Text:
<span class="chemical">MDMA</span> polydrug users show process - specific central executive impairments coupled with impaired social and emotional judgement processes . 