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: Here , 2 cases of urinary bladder retention leading to renal pelvocalyceal dilatation mimicking hydronephrosis as a result of continuous infusion of fentanyl are reported .
### Output Text:
Here , 2 cases of urinary bladder retention leading to renal pelvocalyceal dilatation mimicking hydronephrosis as a result of continuous infusion of <span class="chemical">fentanyl</span> are reported . 