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: The association of the tumor with preexisting hydroureteronephrosis suggests that stasis prolonged and intensified exposure of upper urinary tract epithelium to cyclophosphamide .
### Output Text:
The association of the tumor with preexisting hydroureteronephrosis suggests that stasis prolonged and intensified exposure of upper urinary tract epithelium to <span class="chemical">cyclophosphamide</span> . 