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: Bladder retention of urine as a result of continuous intravenous infusion of fentanyl : 2 case reports .
### Output Text:
Bladder retention of urine as a result of continuous intravenous infusion of <span class="chemical">fentanyl</span> : 2 case reports . 