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: In contrast , two of four consecutive patients who received mesna uroprophylaxis before allogeneic bone marrow transplantation had severe hemorrhagic cystitis for at least 2 weeks .
### Output Text:
In contrast , two of four consecutive patients who received <span class="chemical">mesna</span> uroprophylaxis before allogeneic bone marrow transplantation had severe hemorrhagic cystitis for at least 2 weeks . 