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: High - dose intravenous melphalan followed by peripheral blood stem cell transplant ( PBSCT ) appears to be the most promising therapy , but treatment mortality can be high .
### Output Text:
High - dose intravenous <span class="chemical">melphalan</span> followed by peripheral blood stem cell transplant ( PBSCT ) appears to be the most promising therapy , but treatment mortality can be high . 