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: A busulfan dose calculated on the basis of body surface area , resulting in higher doses in young children , was followed by increased neurotoxicity , close to neurotoxicity incidence observed in adults .
### Output Text:
A <span class="chemical">busulfan</span> dose calculated on the basis of body surface area , resulting in higher doses in young children , was followed by increased neurotoxicity , close to neurotoxicity incidence observed in adults . 