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: The purpose of this study was to determine if PAP formation is a requisite step in APAP - induced nephrotoxicity .
### Output Text:
The purpose of this study was to determine if <span class="chemical">PAP</span> formation is a requisite step in <span class="chemical">APAP</span> - induced nephrotoxicity . 