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: The median age of those patients developing Grade 3 - 4 neutropenia was significantly higher than that of the remaining patients ( 75 years vs . 72 years ; P = 0 . 047 ) .
### Output Text:
The median age of those patients developing Grade 3 - 4 neutropenia was significantly higher than that of the remaining patients ( 75 years vs . 72 years ; P = 0 . 047 ) . 