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: A prospective , randomized , double - blind pilot study to compare the results of stereotactic unilateral pallidotomy and subthalamotomy in advanced idiopathic Parkinson ' s disease ( PD ) refractory to medical treatment was designed .
### Output Text:
A prospective , randomized , double - blind pilot study to compare the results of stereotactic unilateral pallidotomy and subthalamotomy in advanced idiopathic Parkinson ' s disease ( PD ) refractory to medical treatment was designed . 