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: We postulate that by virtue of its direct blocking action on IKr , ketoconazole alone may prolong QT interval and induce TdP .
### Output Text:
We postulate that by virtue of its direct blocking action on IKr , <span class="chemical">ketoconazole</span> alone may prolong QT interval and induce TdP . 