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: In the present study , we have investigated the molecular mechanisms by which female hormones influence cholesterol metabolism in macrophages in response to the HIV protease inhibitor ritonavir .
### Output Text:
In the present study , we have investigated the molecular mechanisms by which female hormones influence <span class="chemical">cholesterol</span> metabolism in macrophages in response to the HIV protease inhibitor <span class="chemical">ritonavir</span> . 