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: Ongoing tubular injury may be a prerequisite for renal injury by melphalan as evidenced by the active urinary sediment .
### Output Text:
Ongoing tubular injury may be a prerequisite for renal injury by <span class="chemical">melphalan</span> as evidenced by the active urinary sediment . 