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: Bilateral visual field constriction was observed in 59 % of patients currently taking VGB , 43 % of patients who previously took VGB , and 24 % of patients with no exposure to VGB .
### Output Text:
Bilateral visual field constriction was observed in 59 % of patients currently taking <span class="chemical">VGB</span> , 43 % of patients who previously took <span class="chemical">VGB</span> , and 24 % of patients with no exposure to <span class="chemical">VGB</span> . 