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: Nephrolithiasis is a complication of acetazolamide but does not preclude its use .
### Output Text:
Nephrolithiasis is a complication of <span class="chemical">acetazolamide</span> but does not preclude its use . 