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 addition , correlations between urinary NGF and PG , and urodynamic parameters in patients with OAB were examined .
### Output Text:
In addition , correlations between urinary NGF and <span class="chemical">PG</span> , and urodynamic parameters in patients with OAB were examined . 