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: RESULTS : Head - up tilt caused systolic orthostatic hypotension which was marked in six of 20 PD patients on selegiline , one of whom lost consciousness with unrecordable blood pressures .
### Output Text:
RESULTS : Head - up tilt caused systolic orthostatic hypotension which was marked in six of 20 PD patients on <span class="chemical">selegiline</span> , one of whom lost consciousness with unrecordable blood pressures . 