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: However , verapamil unresponsiveness discriminates stalk effect ( i . e . , anatomically or functionally inhibited dopaminergic tonus ) from other causes of hyperprolactinemia with varying degrees of responsiveness .
### Output Text:
However , <span class="chemical">verapamil</span> unresponsiveness discriminates stalk effect ( i . e . , anatomically or functionally inhibited dopaminergic tonus ) from other causes of hyperprolactinemia with varying degrees of responsiveness . 