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: Because levodopa increases both phasic and tonic dopaminergic neurotransmission , the critical mechanism mediating the enhancement of learning is unresolved .
### Output Text:
Because <span class="chemical">levodopa</span> increases both phasic and tonic dopaminergic neurotransmission , the critical mechanism mediating the enhancement of learning is unresolved . 