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: AIM : Verapamil stimulation test was previously investigated as a tool for differential diagnosis of hyperprolactinemia , but with conflicting results .
### Output Text:
AIM : <span class="chemical">Verapamil</span> stimulation test was previously investigated as a tool for differential diagnosis of hyperprolactinemia , but with conflicting results . 