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: Possible risk factors included young age , concurrent use of other analgesic agents ( especially ibuprofen ) , preexisting renal disease , a history of kidney stones , a history of gout , a recent increase in activity , a recent increase in sun exposure , and residence in the Sunbelt .
### Output Text:
Possible risk factors included young age , concurrent use of other analgesic agents ( especially <span class="chemical">ibuprofen</span> ) , preexisting renal disease , a history of kidney stones , a history of gout , a recent increase in activity , a recent increase in sun exposure , and residence in the Sunbelt . 