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: In six of the probable cases the neurological disturbance consisted of an acute reversible encephalopathy usually related to the ingestion of a high dose of clioquinol over a short period .
### Output Text:
In six of the probable cases the neurological disturbance consisted of an acute reversible encephalopathy usually related to the ingestion of a high dose of <span class="chemical">clioquinol</span> over a short period . 