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 long - standing secondary hyperparathyroidism , OCT ( 0 . 03 microg / kg ) stabilized serum PTH levels during the first months .
### Output Text:
In long - standing secondary hyperparathyroidism , <span class="chemical">OCT</span> ( 0 . 03 microg / kg ) stabilized serum PTH levels during the first months . 