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: Patients who present with stress incontinence while taking prazosin should change their antihypertensive medication before considering surgery , because their incontinence may resolve spontaneously with a change in drug therapy .
### Output Text:
Patients who present with stress incontinence while taking <span class="chemical">prazosin</span> should change their antihypertensive medication before considering surgery , because their incontinence may resolve spontaneously with a change in drug therapy . 