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: Although the underlying mechanism ( s ) are not well understood , these effects may involve an increase in acetylcholine ( ACh ) levels .
### Output Text:
Although the underlying mechanism ( s ) are not well understood , these effects may involve an increase in <span class="chemical">acetylcholine</span> ( <span class="chemical">ACh</span> ) levels . 