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: Five patients with serial pregnancies with and without propranolol therapy are also examined .
### Output Text:
Five patients with serial pregnancies with and without <span class="chemical">propranolol</span> therapy are also examined . 