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: A rechallenge with androgen produced symptoms of obstructive sleep apnea that abated upon withdrawal of the hormone .
### Output Text:
A rechallenge with <span class="chemical">androgen</span> produced symptoms of obstructive sleep apnea that abated upon withdrawal of the hormone . 