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: This study was undertaken to determine the effects of OCT on serum PTH levels and bone turnover in states of normal or impaired renal function .
### Output Text:
This study was undertaken to determine the effects of <span class="chemical">OCT</span> on serum PTH levels and bone turnover in states of normal or impaired renal function . 