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: The relevance of these features for patients using PDN remains to be elucidated .
### Output Text:
The relevance of these features for patients using <span class="chemical">PDN</span> remains to be elucidated . 