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 case of acute hepatitis induced by zidovudine in a 38 - year - old patient with AIDS is presented .
### Output Text:
A case of acute hepatitis induced by <span class="chemical">zidovudine</span> in a 38 - year - old patient with AIDS is presented . 