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: Three months later the patient was exposed to a single dose of metoprolol , diltiazem , propafenone ( since he had received this drug in the past ) , and sparteine ( as a probe for the debrisoquine / sparteine type polymorphism of oxidative drug metabolism ) .
### Output Text:
Three months later the patient was exposed to a single dose of <span class="chemical">metoprolol</span> , <span class="chemical">diltiazem</span> , <span class="chemical">propafenone</span> ( since he had received this drug in the past ) , and <span class="chemical">sparteine</span> ( as a probe for the <span class="chemical">debrisoquine</span> / <span class="chemical">sparteine</span> type polymorphism of oxidative drug metabolism ) . 