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: We report a case of a 51 - year - old man who developed severe , disabling negative myoclonus of the upper and lower extremities after the infusion of ifosfamide for plasmacytoma .
### Output Text:
We report a case of a 51 - year - old man who developed severe , disabling negative myoclonus of the upper and lower extremities after the infusion of <span class="chemical">ifosfamide</span> for plasmacytoma . 