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: In the present study , the authors determined if treatment with D - MED prevents the muscle rigidity caused by high - dose alfentanil anesthesia in the rat .
### Output Text:
In the present study , the authors determined if treatment with <span class="chemical">D - MED</span> prevents the muscle rigidity caused by high - dose <span class="chemical">alfentanil</span> anesthesia in the rat . 