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 45 - year - old male patient who was on treatment with multiple second - line anti - tuberculous drugs including linezolid and ethambutol for extensively drug - resistant tuberculosis ( XDR - TB ) presented to us with painless progressive loss of vision in both eyes .
### Output Text:
A 45 - year - old male patient who was on treatment with multiple second - line anti - tuberculous drugs including <span class="chemical">linezolid</span> and <span class="chemical">ethambutol</span> for extensively drug - resistant tuberculosis ( XDR - TB ) presented to us with painless progressive loss of vision in both eyes . 