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: A serious complication of ethambutol is an optic neuropathy that impairs visual acuity , contrast sensitivity , and color vision .
### Output Text:
A serious complication of <span class="chemical">ethambutol</span> is an optic neuropathy that impairs visual acuity , contrast sensitivity , and color vision . 