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: The studies suggest that propranolol is a useful drug in selected patients with severe idiopathic orthostatic hypotension .
### Output Text:
The studies suggest that <span class="chemical">propranolol</span> is a useful drug in selected patients with severe idiopathic orthostatic hypotension . 