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: Methadone dose , presence of cytochrome P - 450 3A4 inhibitors , potassium level , and liver function contribute to QT prolongation .
### Output Text:
<span class="chemical">Methadone</span> dose , presence of cytochrome P - 450 3A4 inhibitors , <span class="chemical">potassium</span> level , and liver function contribute to QT prolongation . 