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 most common cause of this syndrome is an interaction between a monoamine oxidase inhibitor ( MAOI ) and a specific serotonin reuptake inhibitor .
### Output Text:
The most common cause of this syndrome is an interaction between a monoamine oxidase inhibitor ( MAOI ) and a specific <span class="chemical">serotonin</span> reuptake inhibitor . 