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: This report presents the clinical , laboratory , and light and electron microscopic observations on a patient with chronic active ( aggressive ) hepatitis caused by the administration of propylthiouracil .
### Output Text:
This report presents the clinical , laboratory , and light and electron microscopic observations on a patient with chronic active ( aggressive ) hepatitis caused by the administration of <span class="chemical">propylthiouracil</span> . 