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: CONCLUSION : The timing of renal injury strongly suggests melphalan as the causative agent .
### Output Text:
CONCLUSION : The timing of renal injury strongly suggests <span class="chemical">melphalan</span> as the causative agent . 