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: At presentation , advanced encephalopathy and cerebral edema were present in 51 ( 76 % ) and 29 ( 41 . 4 % ) patients , respectively .
### Output Text:
At presentation , advanced encephalopathy and cerebral edema were present in 51 ( 76 % ) and 29 ( 41 . 4 % ) patients , respectively . 