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: BACKGROUND : No direct comparisons exist of the renal tolerability of the low - osmolality contrast medium iopamidol with that of the iso - osmolality contrast medium iodixanol in high - risk patients .
### Output Text:
BACKGROUND : No direct comparisons exist of the renal tolerability of the low - osmolality <span class="chemical">contrast medium</span> <span class="chemical">iopamidol</span> with that of the iso - osmolality <span class="chemical">contrast medium</span> <span class="chemical">iodixanol</span> in high - risk patients . 