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: The neuropathy caused by a single dose of isoniazid in rats was studied with a computer - assisted morphometric method .
### Output Text:
The neuropathy caused by a single dose of <span class="chemical">isoniazid</span> in rats was studied with a computer - assisted morphometric method . 