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: A clinical presentation is made of a 2 - 3 year follow - up of six cases of acute renal failure that have been reported earlier .
### Output Text:
A clinical presentation is made of a 2 - 3 year follow - up of six cases of acute renal failure that have been reported earlier . 