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: Hypokalemia , sometimes severe , was observed in some L - dopa - treated parkinsonian patients .
### Output Text:
Hypokalemia , sometimes severe , was observed in some <span class="chemical">L - dopa</span> - treated parkinsonian patients . 