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 significant decrease was observed in the activity of the myocardial marker enzymes with a concomitant increase in their activity in serum .
### Output Text:
A significant decrease was observed in the activity of the myocardial marker enzymes with a concomitant increase in their activity in serum . 