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 healthy 17 - year - old male received standard intermittent doses of pethidine via a patient - controlled analgesia ( PCA ) pump for management of postoperative pain control .
### Output Text:
A healthy 17 - year - old male received standard intermittent doses of <span class="chemical">pethidine</span> via a patient - controlled analgesia ( PCA ) pump for management of postoperative pain control . 