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 objective of this study was to report our experience concerning the effectiveness of the prophylactic administration of lamivudine in hepatitis B virus surface antigen ( HBs Ag ) positive patients with rheumatologic disease .
### Output Text:
The objective of this study was to report our experience concerning the effectiveness of the prophylactic administration of <span class="chemical">lamivudine</span> in <span class="chemical">hepatitis B virus surface antigen</span> ( <span class="chemical">HBs Ag</span> ) positive patients with rheumatologic disease . 