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: Since plasma pharmacokinetic studies showed a faster busulfan clearance in children than in adults , this new dose may approximate more closely the adult systemic exposure obtained after the usual 16 - mg / kg total dose , with potential inferences in terms of anticancer or myeloablative effects .
### Output Text:
Since plasma pharmacokinetic studies showed a faster <span class="chemical">busulfan</span> clearance in children than in adults , this new dose may approximate more closely the adult systemic exposure obtained after the usual 16 - mg / kg total dose , with potential inferences in terms of anticancer or myeloablative effects . 