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: BACKGROUND AND PURPOSE : Cerebral microbleeds ( MB ) are potential risk factors for intracerebral hemorrhage ( ICH ) , but it is unclear if they are a contraindication to using antithrombotic drugs .
### Output Text:
BACKGROUND AND PURPOSE : Cerebral microbleeds ( MB ) are potential risk factors for intracerebral hemorrhage ( ICH ) , but it is unclear if they are a contraindication to using antithrombotic drugs . 