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 imagery resembled visual disturbances previously experienced as " flashbacks " related to prior LSD consumption .
### Output Text:
This imagery resembled visual disturbances previously experienced as " flashbacks " related to prior <span class="chemical">LSD</span> consumption . 