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: For this reason , each clinician should recognize this entity in the differential diagnosis of PB - related asymptomatic chronic hepatic enzyme dysfunction .
### Output Text:
For this reason , each clinician should recognize this entity in the differential diagnosis of <span class="chemical">PB</span> - related asymptomatic chronic hepatic enzyme dysfunction . 