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: RESULTS : Following intravenous PAN urine volume and urine NAG activity increased significantly by day two , but returned to normal by day four .
### Output Text:
RESULTS : Following intravenous <span class="chemical">PAN</span> urine volume and urine NAG activity increased significantly by day two , but returned to normal by day four . 