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: 18 patients , referred to a radiocontrast study , considered at risk because of preexisting renal insufficiency , were enrolled in a prospective , randomized , controlled trial , performed at the secondary care center of a 1 , 100 - bed private university hospital .
### Output Text:
18 patients , referred to a radiocontrast study , considered at risk because of preexisting renal insufficiency , were enrolled in a prospective , randomized , controlled trial , performed at the secondary care center of a 1 , 100 - bed private university hospital . 