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: Whereas muscular rigidity is a well - known side effect that is associated with high - dose fentanyl anesthesia , a paucity of information exists with regard to its underlying mechanism ( s ) .
### Output Text:
Whereas muscular rigidity is a well - known side effect that is associated with high - dose <span class="chemical">fentanyl</span> anesthesia , a paucity of information exists with regard to its underlying mechanism ( s ) . 