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: Antituberculosis therapy ( ATT ) - associated acute liver failure ( ATT - ALF ) is the commonest drug - induced ALF in South Asia .
### Output Text:
<span class="chemical">Antituberculosis</span> therapy ( ATT ) - associated acute liver failure ( ATT - ALF ) is the commonest drug - induced ALF in South Asia . 