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: Myotonia congenita ( MC ) is caused by a defect in the skeletal muscle chloride channel function , which may cause sustained membrane depolarisation .
### Output Text:
Myotonia congenita ( MC ) is caused by a defect in the skeletal muscle <span class="chemical">chloride</span> channel function , which may cause sustained membrane depolarisation . 