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: This study investigated the visual defects associated with the antiepileptic drug vigabatrin ( VGB ) .
### Output Text:
This study investigated the visual defects associated with the antiepileptic drug <span class="chemical">vigabatrin</span> ( <span class="chemical">VGB</span> ) . 