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: This retrospective study examines the incidence and treatment of ESRD and chronic renal failure ( CRF ) in OLTX patients .
### Output Text:
This retrospective study examines the incidence and treatment of ESRD and chronic renal failure ( CRF ) in OLTX patients . 