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: Cases were patients who developed hyperkalemia ( K ( + ) > 5 . 0 mEq / L ) or renal insufficiency ( Cr > or = 2 . 5 mg / dL ) , and they were compared to 2 randomly selected controls per case .
### Output Text:
Cases were patients who developed hyperkalemia ( <span class="chemical">K</span> ( + ) > 5 . 0 mEq / L ) or renal insufficiency ( <span class="chemical">Cr</span> > or = 2 . 5 mg / dL ) , and they were compared to 2 randomly selected controls per case . 