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: The aim of the present study is to examine whether myocardial ischemia due to coronary spasm is induced by dobutamine .
### Output Text:
The aim of the present study is to examine whether myocardial ischemia due to coronary spasm is induced by <span class="chemical">dobutamine</span> . 