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 patient ' s clinical course is described and correlated with initial urodynamic studies while on prazosin and subsequent studies while taking verapamil .
### Output Text:
The patient ' s clinical course is described and correlated with initial urodynamic studies while on <span class="chemical">prazosin</span> and subsequent studies while taking <span class="chemical">verapamil</span> . 