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: Frequency of appearance of myeloperoxidase - antineutrophil cytoplasmic antibody ( MPO - ANCA ) in Graves ' disease patients treated with propylthiouracil and the relationship between MPO - ANCA and clinical manifestations .
### Output Text:
Frequency of appearance of myeloperoxidase - antineutrophil cytoplasmic antibody ( MPO - ANCA ) in Graves ' disease patients treated with <span class="chemical">propylthiouracil</span> and the relationship between MPO - ANCA and clinical manifestations . 