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: We now advocate that before age 50 years testosterone be determined only in cases of low sexual desire and abnormal physical examination but that it be measured in all men older than 50 years .
### Output Text:
We now advocate that before age 50 years <span class="chemical">testosterone</span> be determined only in cases of low sexual desire and abnormal physical examination but that it be measured in all men older than 50 years . 