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 case of genuine stress incontinence due to prazosin , a common antihypertensive drug , is presented .
### Output Text:
A case of genuine stress incontinence due to <span class="chemical">prazosin</span> , a common antihypertensive drug , is presented . 