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: We characterize the clinical , neurophysiological , and neuroimaging findings in a 72 - year - old man who developed visual loss and paresthesias after 11 weeks of exposure to a supratherapeutic dose of ethambutol .
### Output Text:
We characterize the clinical , neurophysiological , and neuroimaging findings in a 72 - year - old man who developed visual loss and paresthesias after 11 weeks of exposure to a supratherapeutic dose of <span class="chemical">ethambutol</span> . 