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: Ximelagatran , an oral direct thrombin inhibitor , was found to be as efficient as vitamin K antagonist drugs in the prevention of embolic events , but has been recently withdrawn because of abnormal liver function tests .
### Output Text:
<span class="chemical">Ximelagatran</span> , an oral direct thrombin inhibitor , was found to be as efficient as <span class="chemical">vitamin K</span> antagonist drugs in the prevention of embolic events , but has been recently withdrawn because of abnormal liver function tests . 