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: METHODS : We conducted a randomized , double - blind , prospective , multicenter study comparing the nephrotoxic effects of an iso - osmolar , dimeric , nonionic contrast medium , iodixanol , with those of a low - osmolar , nonionic , monomeric contrast medium , iohexol .
### Output Text:
METHODS : We conducted a randomized , double - blind , prospective , multicenter study comparing the nephrotoxic effects of an iso - osmolar , dimeric , nonionic contrast medium , <span class="chemical">iodixanol</span> , with those of a low - osmolar , nonionic , monomeric contrast medium , <span class="chemical">iohexol</span> . 