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: PURPOSE : NGF and PGs in the bladder can be affected by pathological changes in the bladder and these changes can be detected in urine .
### Output Text:
PURPOSE : NGF and <span class="chemical">PGs</span> in the bladder can be affected by pathological changes in the bladder and these changes can be detected in urine . 