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: After adjusting for the stage of sexual development and height and BMI z scores , serum prolactin was negatively associated with trabecular volumetric BMD at the ultradistal radius ( P < . 03 ) .
### Output Text:
After adjusting for the stage of sexual development and height and BMI z scores , serum prolactin was negatively associated with trabecular volumetric BMD at the ultradistal radius ( P < . 03 ) . 