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 thoraco - abdominal rigidity leading to respiratory failure is described in the post - operative period in an elderly patient who received a moderate dose of fentanyl .
### Output Text:
A case of thoraco - abdominal rigidity leading to respiratory failure is described in the post - operative period in an elderly patient who received a moderate dose of <span class="chemical">fentanyl</span> . 