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: Assessment of retinal function revealed abnormal responses in 48 % of current VGB users and 22 % of prior VGB users , but in none of the patients without previous exposure to VGB .
### Output Text:
Assessment of retinal function revealed abnormal responses in 48 % of current <span class="chemical">VGB</span> users and 22 % of prior <span class="chemical">VGB</span> users , but in none of the patients without previous exposure to <span class="chemical">VGB</span> . 