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: CLINICAL PICTURE : Three patients with ethambutol - associated toxic optic neuropathy are described .
### Output Text:
CLINICAL PICTURE : Three patients with <span class="chemical">ethambutol</span> - associated toxic optic neuropathy are described . 