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: Patients Fifty subjects signed informed consent and 41 underwent the frequently sampled intravenous glucose tolerance test .
### Output Text:
Patients Fifty subjects signed informed consent and 41 underwent the frequently sampled intravenous <span class="chemical">glucose</span> tolerance test . 