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 mechanism by which omeprazole caused the patient ' s hemolytic anemia is uncertain , but physicians should be alerted to this possible adverse effect .
### Output Text:
The mechanism by which <span class="chemical">omeprazole</span> caused the patient ' s hemolytic anemia is uncertain , but physicians should be alerted to this possible adverse effect . 