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 41 - year - old woman with a strong history of postoperative nausea and vomiting presented for abdominal hysterectomy 3 months after a previous anaesthetic where ondansetron prophylaxis had been used .
### Output Text:
A 41 - year - old woman with a strong history of postoperative nausea and vomiting presented for abdominal hysterectomy 3 months after a previous anaesthetic where <span class="chemical">ondansetron</span> prophylaxis had been used . 