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: BACKGROUND : Calcitriol therapy suppresses serum levels of parathyroid hormone ( PTH ) in patients with renal failure but has several drawbacks , including hypercalcemia and / or marked suppression of bone turnover , which may lead to adynamic bone disease .
### Output Text:
BACKGROUND : <span class="chemical">Calcitriol</span> therapy suppresses serum levels of parathyroid hormone ( PTH ) in patients with renal failure but has several drawbacks , including hypercalcemia and / or marked suppression of bone turnover , which may lead to adynamic bone disease . 