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: This was accepted as evidence for propranolol being the cause of this conduction disorder .
### Output Text:
This was accepted as evidence for <span class="chemical">propranolol</span> being the cause of this conduction disorder . 