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: Binasal visual field defects are not specific to vigabatrin .
### Output Text:
Binasal visual field defects are not specific to <span class="chemical">vigabatrin</span> . 