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: Bilateral optic neuropathy due to combined ethambutol and isoniazid treatment .
### Output Text:
Bilateral optic neuropathy due to combined <span class="chemical">ethambutol</span> and <span class="chemical">isoniazid</span> treatment . 