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 total of 261 adverse ocular reactions occurred in 237 patients who received isotretinoin , a commonly used drug in the treatment of severe cystic acne .
### Output Text:
A total of 261 adverse ocular reactions occurred in 237 patients who received <span class="chemical">isotretinoin</span> , a commonly used drug in the treatment of severe cystic acne . 