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 second echocardiography was performed ( median interval : 13 months ) after pergolide withdrawal ( n = 10 patients ) .
### Output Text:
A second echocardiography was performed ( median interval : 13 months ) after <span class="chemical">pergolide</span> withdrawal ( n = 10 patients ) . 