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: Structural and functional measurements indicate that androgens exert a permissive or necessary action on the structural configuration of the oropharynx that predisposes to obstruction during sleep .
### Output Text:
Structural and functional measurements indicate that <span class="chemical">androgens</span> exert a permissive or necessary action on the structural configuration of the oropharynx that predisposes to obstruction during sleep . 