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: Verapamil stimulation test in hyperprolactinemia : loss of prolactin response in anatomic or functional stalk effect .
### Output Text:
<span class="chemical">Verapamil</span> stimulation test in hyperprolactinemia : loss of prolactin response in anatomic or functional stalk effect . 