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: The purpose of the present study was to investigate the changes in plasma and urinary lipoproteins during the administration of puromycin aminonucleoside ( 20 mg / kg for 7 days ) and the subsequent development of nephrotic syndrome .
### Output Text:
The purpose of the present study was to investigate the changes in plasma and urinary lipoproteins during the administration of <span class="chemical">puromycin aminonucleoside</span> ( 20 mg / kg for 7 days ) and the subsequent development of nephrotic syndrome . 