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 73 - year - old woman died of aplastic anemia less than two months after undergoing cataract extraction and beginning topical therapy with chloramphenicol .
### Output Text:
A 73 - year - old woman died of aplastic anemia less than two months after undergoing cataract extraction and beginning topical therapy with <span class="chemical">chloramphenicol</span> . 