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: In 13 patients we observed the effects of HCTZ therapy ( 100 mg daily ) on the occurrence of PVC ' s during rest as well as during static and dynamic exercise .
### Output Text:
In 13 patients we observed the effects of <span class="chemical">HCTZ</span> therapy ( 100 mg daily ) on the occurrence of PVC ' s during rest as well as during static and dynamic exercise . 