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 risk of venous thromboembolism in women prescribed cyproterone acetate in combination with ethinyl estradiol : a nested cohort analysis and case - control study .
### Output Text:
The risk of venous thromboembolism in women prescribed <span class="chemical">cyproterone acetate</span> in combination with <span class="chemical">ethinyl estradiol</span> : a nested cohort analysis and case - control study . 