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: All the patients were skin test negative to BPO ; 49 of 51 ( 96 % ) were also negative to MDM , and 44 of 46 ( 96 % ) to PG .
### Output Text:
All the patients were skin test negative to <span class="chemical">BPO</span> ; 49 of 51 ( 96 % ) were also negative to MDM , and 44 of 46 ( 96 % ) to <span class="chemical">PG</span> . 