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: Nondyskinetic patients , but not the dyskinetic ones , showed less oropharyngeal swallowing efficiency ( OPSE ) for liquid food than controls ( Dunnett , P = 0 . 02 ) .
### Output Text:
Nondyskinetic patients , but not the dyskinetic ones , showed less oropharyngeal swallowing efficiency ( OPSE ) for liquid food than controls ( Dunnett , P = 0 . 02 ) . 